The Student's Elements of Geology
Sir Charles Lyell
Part 2 out of 14
impressions, but it has only one in the adult state.)
(FIGURE 24. Planorbis euomphalus, Sowerby; fossil. Isle of Wight.)
(FIGURE 25. Limnaea longiscala, Brongniart; fossil. Isle of Wight.)
(FIGURE 26. Paludina lenta, Brand.; fossil. Isle of Wight.)
(FIGURE 27. Succinea amphibia, Drap. (S. putris, L.); fossil. Loess, Rhine.)
(FIGURE 28. Ancylus velletia (A. elegans), Sowerby; fossil. Isle of Wight.)
(FIGURE 29. Valvata piscinalis, Mull.; fossil. Grays, Essex.)
(FIGURE 30. Physa hypnorum, Linne; recent. Isle of Wight.)
(FIGURE 31. Auricula; recent. Ava.)
(FIGURE 32. Melania inquinata, Def. Paris basin.)
(FIGURE 33. Physa columnaris, Desh. Paris basin.)
(FIGURE 34. Melanopsis buccinoidea, Ferr.; recent. Asia.)
The univalve shells most characteristic of fresh-water deposits are, Planorbis,
Limnaea, and Paludina. (See Figures 24-26.) But to these are occasionally added
Physa, Succinea, Ancylus, Valvata, Melanopsis, Melania, Potamides, and Neritina
(see Figures 27-34), the four last being usually found in estuaries.
(FIGURE 35. Neritina globulus, Def. Paris basin.)
(FIGURE 36. Nerita granulosa, Desh. Paris basin.)
Some naturalists include Neritina (Figure 35) and the marine Nerita (Figure 36)
in the same genus, it being scarcely possible to distinguish the two by good
generic characters. But, as a general rule, the fluviatile species are smaller,
smoother, and more globular than the marine; and they have never, like the
Neritae, the inner margin of the outer lip toothed or crenulated. (See Figure
(FIGURE 37. Potamides cinctus, Sowerby. Paris basin.)
The Potamides inhabit the mouths of rivers in warm latitudes, and are
distinguishable from the marine Cerithia by their orbicular and multispiral
opercula. The genus Auricula (Figure 31) is amphibious, frequenting swamps and
marshes within the influence of the tide.
(FIGURE 38. Helix Turonensis, Desh.; faluns, Touraine.)
(FIGURE 39. Cyclostoma elegans, Mull.; Loess.)
(FIGURE 40. Pupa tridens, Drap.; Loess.)
(FIGURE 41. Clausilia bidens, Drap.; Loess.)
(FIGURE 42. Bulimus lubricus, Mull.; Loess, Rhine.)
The terrestrial shells are all univalves. The most important genera among these,
both in a recent and fossil state, are Helix (Figure 38), Cyclostoma (Figure
39), Pupa (Figure 40), Clausilia (Figure 41) Bulimus (Figure 42), Glandina and
(FIGURE 43. Ampullaria glauca, from the Jumna.)
Ampullaria (Figure 43) is another genus of shells inhabiting rivers and ponds in
hot countries. Many fossil species formerly referred to this genus, and which
have been met with chiefly in marine formations, are now considered by
conchologists to belong to Natica and other marine genera.
(FIGURE 44. Pleurotoma exorta, Brand. Upper and Middle Eocene. Barton and
(FIGURE 45. Ancillaria subulata, Sowerby. Barton clay. Eocene.)
All univalve shells of land and fresh-water species, with the exception of
Melanopsis (Figure 34), and Achatina, which has a slight indentation, have
entire mouths; and this circumstance may often serve as a convenient rule for
distinguishing fresh-water from marine strata; since, if any univalves occur of
which the mouths are not entire, we may presume that the formation is marine.
The aperture is said to be entire in such shells as the fresh-water Ampullaria
and the land-shells (Figures 38-42), when its outline is not interrupted by an
indentation or notch, such as that seen at b in Ancillaria (Figure 45); or is
not prolonged into a canal, as that seen at a in Pleurotoma (Figure 44).
The mouths of a large proportion of the marine univalves have these notches or
canals, and almost all species are carnivorous; whereas nearly all testacea
having entire mouths are plant-eaters, whether the species be marine, fresh-
water, or terrestrial.
There is, however, one genus which affords an occasional exception to one of the
above rules. The Potamides (Figure 37), a subgenus of Cerithium, although
provided with a short canal, comprises some species which inhabit salt, others
brackish, and others fresh-water, and they are said to be all plant-eaters.
Among the fossils very common in fresh-water deposits are the shells of Cypris,
a minute bivalve crustaceous animal. (For figures of fossil species of Purbeck
see below, Chapter 19.) Many minute living species of this genus swarm in lakes
and stagnant pools in Great Britain; but their shells are not, if considered
separately, conclusive as to the fresh-water origin of a deposit, because the
majority of species in another kindred genus of the same order, the Cytherina of
Lamarck, inhabit salt-water; and, although the animal differs slightly, the
shell is scarcely distinguishable from that of the Cypris.
FRESH-WATER FOSSIL PLANTS.
(FIGURE 46. Chara medicaginula; fossil. Upper Eocene, Isle of Wight.)
The seed-vessels and stems of Chara, a genus of aquatic plants, are very
frequent in fresh-water strata. These seed-vessels were called, before their
true nature was known, gyrogonites, and were supposed to be foraminiferous
shells. (See Figure 46, a.)
(FIGURE 47. Chara elastica; recent, Italy.
a. Sessile seed-vessel between the divisions of the leaves of the female plant.
b. Magnified transverse section of a branch, with five seed-vessels, seen from
The Charae inhabit the bottom of lakes and ponds, and flourish mostly where the
water is charged with carbonate of lime. Their seed-vessels are covered with a
very tough integument, capable of resisting decomposition; to which circumstance
we may attribute their abundance in a fossil state. Figure 47 represents a
branch of one of many new species found by Professor Amici in the lakes of
Northern Italy. The seed-vessel in this plant is more globular than in the
British Charae, and therefore more nearly resembles in form the extinct fossil
species found in England, France, and other countries. The stems, as well as the
seed-vessels, of these plants occur both in modern shell-marl and in ancient
fresh-water formations. They are generally composed of a large central tube
surrounded by smaller ones; the whole stem being divided at certain intervals by
transverse partitions or joints. (See b, Figure 46.)
It is not uncommon to meet with layers of vegetable matter, impressions of
leaves, and branches of trees, in strata containing fresh-water shells; and we
also find occasionally the teeth and bones of land quadrupeds, of species now
unknown. The manner in which such remains are occasionally carried by rivers
into lakes, especially during floods, has been fully treated of in the
"Principles of Geology."
FRESH-WATER AND MARINE FISH.
The remains of fish are occasionally useful in determining the fresh-water
origin of strata. Certain genera, such as carp, perch, pike, and loach
(Cyprinus, Perca, Esox, and Cobitis), as also Lebias, being peculiar to fresh-
water. Other genera contain some fresh-water and some marine species, as Cottus,
Mugil, and Anguilla, or eel. The rest are either common to rivers and the sea,
as the salmon; or are exclusively characteristic of salt-water. The above
observations respecting fossil fishes are applicable only to the more modern or
tertiary deposits; for in the more ancient rocks the forms depart so widely from
those of existing fishes, that it is very difficult, at least in the present
state of science, to derive any positive information from ichthyolites
respecting the element in which strata were deposited.
The alternation of marine and fresh-water formations, both on a small and large
scale, are facts well ascertained in geology. When it occurs on a small scale,
it may have arisen from the alternate occupation of certain spaces by river-
water and the sea; for in the flood season the river forces back the ocean and
freshens it over a large area, depositing at the same time its sediment; after
which the salt-water again returns, and, on resuming its former place, brings
with it sand, mud, and marine shells.
There are also lagoons at the mouth of many rivers, as the Nile and Mississippi,
which are divided off by bars of sand from the sea, and which are filled with
salt and fresh water by turns. They often communicate exclusively with the river
for months, years, or even centuries; and then a breach being made in the bar of
sand, they are for long periods filled with salt-water.
The Lym-Fiord in Jutland offers an excellent illustration of analogous changes;
for, in the course of the last thousand years, the western extremity of this
long frith, which is 120 miles in length, including its windings, has been four
times fresh and four times salt, a bar of sand between it and the ocean having
been often formed and removed. The last irruption of salt water happened in
1824, when the North Sea entered, killing all the fresh-water shells, fish, and
plants; and from that time to the present, the sea-weed Fucus vesiculosus,
together with oysters and other marine mollusca, have succeeded the Cyclas,
Lymnaea, Paludina, and Charae. (See Principles Index "Lym-Fiord.")
But changes like these in the Lym-Fiord, and those before mentioned as occurring
at the mouths of great rivers, will only account for some cases of marine
deposits of partial extent resting on fresh-water strata. When we find, as in
the south-east of England (Chapter 18), a great series of fresh-water beds, 1000
feet in thickness, resting upon marine formations and again covered by other
rocks, such as the Cretaceous, more than 1000 feet thick, and of deep-sea
origin, we shall find it necessary to seek for a different explanation of the
CONSOLIDATION OF STRATA AND PETRIFACTION OF FOSSILS.
Chemical and Mechanical Deposits.
Cementing together of Particles.
Hardening by Exposure to Air.
Consolidating Effects of Pressure.
Mineralization of Organic Remains.
Impressions and Casts: how formed.
Precipitation of Stony Matter most rapid where Putrefaction is going on.
Sources of Lime and Silex in Solution.
Having spoken in the preceding chapters of the characters of sedimentary
formations, both as dependent on the deposition of inorganic matter and the
distribution of fossils, I may next treat of the consolidation of stratified
rocks, and the petrifaction of imbedded organic remains.
CHEMICAL AND MECHANICAL DEPOSITS.
A distinction has been made by geologists between deposits of a mechanical, and
those of a chemical, origin. By the name mechanical are designated beds of mud,
sand, or pebbles produced by the action of running water, also accumulations of
stones and scoriae thrown out by a volcano, which have fallen into their present
place by the force of gravitation. But the matter which forms a chemical deposit
has not been mechanically suspended in water, but in a state of solution until
separated by chemical action. In this manner carbonate of lime is occasionally
precipitated upon the bottom of lakes in a solid form, as may be well seen in
many parts of Italy, where mineral springs abound, and where the calcareous
stone, called travertin, is deposited. In these springs the lime is usually held
in solution by an excess of carbonic acid, or by heat if it be a hot spring,
until the water, on issuing from the earth, cools or loses part of its acid. The
calcareous matter then falls down in a solid state, incrusting shells, fragments
of wood and leaves, and binding them together.
That similar travertin is formed at some points in the bed of the sea where
calcareous springs issue can not be doubted, but as a general rule the quantity
of lime, according to Bischoff, spread through the waters of the ocean is very
small, the free carbonic acid gas in the same waters being five times as much as
is necessary to keep the lime in a fluid state. Carbonate of lime, therefore,
can rarely be precipitated at the bottom of the sea by chemical action alone,
but must be produced by vital agency as in the case of coral reefs.
In such reefs, large masses of limestone are formed by the stony skeletons of
zoophytes; and these, together with shells, become cemented together by
carbonate of lime, part of which is probably furnished to the sea-water by the
decomposition of dead corals. Even shells, of which the animals are still living
on these reefs, are very commonly found to be incrusted over with a hard coating
If sand and pebbles are carried by a river into the sea, and these are bound
together immediately by carbonate of lime, the deposit may be described as of a
mixed origin, partly chemical, and partly mechanical.
Now, the remarks already made in Chapter 2 on the original horizontality of
strata are strictly applicable to mechanical deposits, and only partially to
those of a mixed nature. Such as are purely chemical may be formed on a very
steep slope, or may even incrust the vertical walls of a fissure, and be of
equal thickness throughout; but such deposits are of small extent, and for the
most part confined to vein-stones.
CONSOLIDATION OF STRATA.
It is chiefly in the case of calcareous rocks that solidification takes place at
the time of deposition. But there are many deposits in which a cementing process
comes into operation long afterwards. We may sometimes observe, where the water
of ferruginous or calcareous springs has flowed through a bed of sand or gravel,
that iron or carbonate of lime has been deposited in the interstices between the
grains or pebbles, so that in certain places the whole has been bound together
into a stone, the same set of strata remaining in other parts loose and
Proofs of a similar cementing action are seen in a rock at Kelloway, in
Wiltshire. A peculiar band of sandy strata belonging to the group called Oolite
by geologists may be traced through several counties, the sand being for the
most part loose and unconsolidated, but becoming stony near Kelloway. In this
district there are numerous fossil shells which have decomposed, having for the
most part left only their casts. The calcareous matter hence derived has
evidently served, at some former period, as a cement to the siliceous grains of
sand, and thus a solid sandstone has been produced. If we take fragments of many
other argillaceous grits, retaining the casts of shells, and plunge them into
dilute muriatic or other acid, we see them immediately changed into common sand
and mud; the cement of lime, derived from the shells, having been dissolved by
Traces of impressions and casts are often extremely faint. In some loose sands
of recent date we meet with shells in so advanced a stage of decomposition as to
crumble into powder when touched. It is clear that water percolating such strata
may soon remove the calcareous matter of the shell; and unless circumstances
cause the carbonate of lime to be again deposited, the grains of sand will not
be cemented together; in which case no memorial of the fossil will remain.
In what manner silex and carbonate of lime may become widely diffused in small
quantities through the waters which permeate the earth's crust will be spoken of
presently, when the petrifaction of fossil bodies is considered; but I may
remark here that such waters are always passing in the case of thermal springs
from hotter to colder parts of the interior of the earth; and, as often as the
temperature of the solvent is lowered, mineral matter has a tendency to separate
from it and solidify. Thus a stony cement is often supplied to sand, pebbles, or
any fragmentary mixture. In some conglomerates, like the pudding-stone of
Hertfordshire (a Lower Eocene deposit), pebbles of flint and grains of sand are
united by a siliceous cement so firmly, that if a block be fractured, the rent
passes as readily through the pebbles as through the cement.
It is probable that many strata became solid at the time when they emerged from
the waters in which they were deposited, and when they first formed a part of
the dry land. A well-known fact seems to confirm this idea: by far the greater
number of the stones used for building and road-making are much softer when
first taken from the quarry than after they have been long exposed to the air;
and these, when once dried, may afterwards be immersed for any length of time in
water without becoming soft again. Hence it is found desirable to shape the
stones which are to be used in architecture while they are yet soft and wet, and
while they contain their "quarry-water," as it is called; also to break up stone
intended for roads when soft, and then leave it to dry in the air for months
that it may harden. Such induration may perhaps be accounted for by supposing
the water, which penetrates the minutest pores of rocks, to deposit, on
evaporation, carbonate of lime, iron, silex, and other minerals previously held
in solution, and thereby to fill up the pores partially. These particles, on
crystallising, would not only be themselves deprived of freedom of motion, but
would also bind together other portions of the rock which before were loosely
aggregated. On the same principle wet sand and mud become as hard as stone when
frozen; because one ingredient of the mass, namely, the water, has crystallised,
so as to hold firmly together all the separate particles of which the loose mud
and sand were composed.
Dr. MacCulloch mentions a sandstone in Skye, which may be moulded like dough
when first found; and some simple minerals, which are rigid and as hard as glass
in our cabinets, are often flexible and soft in their native beds: this is the
case with asbestos, sahlite, tremolite, and chalcedony, and it is reported also
to happen in the case of the beryl. (Dr. MacCulloch System of Geology volume 1
The marl recently deposited at the bottom of Lake Superior, in North America, is
soft, and often filled with fresh-water shells; but if a piece be taken up and
dried, it becomes so hard that it can only be broken by a smart blow of the
hammer. If the lake, therefore, was drained, such a deposit would be found to
consist of strata of marlstone, like that observed in many ancient European
formations, and, like them, containing fresh-water shells.
(FIGURE 48. Calcareous nodules in Lias.)
It is probable that some of the heterogeneous materials which rivers transport
to the sea may at once set under water, like the artificial mixture called
pozzolana, which consists of fine volcanic sand charged with about twenty per
cent of oxide of iron, and the addition of a small quantity of lime. This
substance hardens, and becomes a solid stone in water, and was used by the
Romans in constructing the foundations of buildings in the sea. Consolidation in
such cases is brought about by the action of chemical affinity on finely
comminuted matter previously suspended in water. After deposition similar
particles seem often to exert a mutual attraction on each other, and congregate
together in particular spots, forming lumps, nodules, and concretions. Thus in
many argillaceous deposits there are calcareous balls, or spherical concretions,
ranged in layers parallel to the general stratification; an arrangement which
took place after the shale or marl had been thrown down in successive laminae;
for these laminae are often traceable through the concretions, remaining
parallel to those of the surrounding unconsolidated rock. (See Figure 48.) Such
nodules of limestone have often a shell or other foreign body in the centre.
(FIGURE 49. Spheroidal concretions in magnesian limestone.)
Among the most remarkable examples of concretionary structure are those
described by Professor Sedgwick as abounding in the magnesian limestone of the
north of England. The spherical balls are of various sizes, from that of a pea
to a diameter of several feet, and they have both a concentric and radiated
structure, while at the same time the laminae of original deposition pass
uninterruptedly through them. In some cliffs this limestone resembles a great
irregular pile of cannon-balls. Some of the globular masses have their centre in
one stratum, while a portion of their exterior passes through to the stratum
above or below. Thus the larger spheroid in the section (Figure 49) passes from
the stratum b upward into a. In this instance we must suppose the deposition of
a series of minor layers, first forming the stratum b, and afterwards the
incumbent stratum a; then a movement of the particles took place, and the
carbonates of lime and magnesia separated from the more impure and mixed matter
forming the still unconsolidated parts of the stratum. Crystallisation,
beginning at the centre, must have gone on forming concentric coats around the
original nucleus without interfering with the laminated structure of the rock.
(FIGURE 50. Section through strata of grit.)
When the particles of rocks have been thus rearranged by chemical forces, it is
sometimes difficult or impossible to ascertain whether certain lines of division
are due to original deposition or to the subsequent aggregation of several
particles. Thus suppose three strata of grit, A, B, C, are charged unequally
with calcareous matter, and that B is the most calcareous. If consolidation
takes place in B, the concretionary action may spread upward into a part of A,
where the carbonate of lime is more abundant than in the rest; so that a mass, d
e f, forming a portion of the superior stratum, becomes united with B into one
solid mass of stone. The original line of division, d e, being thus effaced, the
line d f would generally be considered as the surface of the bed B, though not
strictly a true plane of stratification. (Figure 50.)
PRESSURE AND HEAT.
When sand and mud sink to the bottom of a deep sea, the particles are not
pressed down by the enormous weight of the incumbent ocean; for the water, which
becomes mingled with the sand and mud, resists pressure with a force equal to
that of the column of fluid above. The same happens in regard to organic remains
which are filled with water under great pressure as they sink, otherwise they
would be immediately crushed to pieces and flattened. Nevertheless, if the
materials of a stratum remain in a yielding state, and do not set or solidify,
they will be gradually squeezed down by the weight of other materials
successively heaped upon them, just as soft clay or loose sand on which a house
is built may give way. By such downward pressure particles of clay, sand, and
marl may become packed into a smaller space, and be made to cohere together
Analogous effects of condensation may arise when the solid parts of the earth's
crust are forced in various directions by those mechanical movements hereafter
to be described, by which strata have been bent, broken, and raised above the
level of the sea. Rocks of more yielding materials must often have been forced
against others previously consolidated, and may thus by compression have
acquired a new structure. A recent discovery may help us to comprehend how fine
sediment derived from the detritus of rocks may be solidified by mere pressure.
The graphite or "black lead" of commerce having become very scarce, Mr.
Brockedon contrived a method by which the dust of the purer portions of the
mineral found in Borrowdale might be recomposed into a mass as dense and compact
as native graphite. The powder of graphite is first carefully prepared and freed
from air, and placed under a powerful press on a strong steel die, with air-
tight fittings. It is then struck several blows, each of a power of 1000 tons;
after which operation the powder is so perfectly solidified that it can be cut
for pencils, and exhibits when broken the same texture as native graphite.
But the action of heat at various depths in the earth is probably the most
powerful of all causes in hardening sedimentary strata. To this subject I shall
refer again when treating of the metamorphic rocks, and of the slaty and jointed
MINERALISATION OF ORGANIC REMAINS.
(FIGURE 51. Phasianella Heddingtonensis, and cast of the same. Coral Rag.)
(FIGURE 52. Pleurotomaria Anglica, and cast. Lias.)
The changes which fossil organic bodies have undergone since they were first
imbedded in rocks, throw much light on the consolidation of strata. Fossil
shells in some modern deposits have been scarcely altered in the course of
centuries, having simply lost a part of their animal matter. But in other cases
the shell has disappeared, and left an impression only of its exterior, or,
secondly, a cast of its interior form, or, thirdly, a cast of the shell itself,
the original matter of which has been removed. These different forms of
fossilisation may easily be understood if we examine the mud recently thrown out
from a pond or canal in which there are shells. If the mud be argillaceous, it
acquires consistency on drying, and on breaking open a portion of it we find
that each shell has left impressions of its external form. If we then remove the
shell itself, we find within a solid nucleus of clay, having the form of the
interior of the shell. This form is often very different from that of the outer
shell. Thus a cast such as a, Figure 51, commonly called a fossil screw, would
never be suspected by an inexperienced conchologist to be the internal shape of
the fossil univalve, b, Figure 51. Nor should we have imagined at first sight
that the shell a and the cast b, Figure 52, belong to one and the same fossil.
The reader will observe, in the last-mentioned figure (b, Figure 52), that an
empty space shaded dark, which the SHELL ITSELF once occupied, now intervenes
between the enveloping stone and the cast of the smooth interior of the whorls.
In such cases the shell has been dissolved and the component particles removed
by water percolating the rock. If the nucleus were taken out, a hollow mould
would remain, on which the external form of the shell with its tubercles and
striae, as seen in a, Figure 52, would be seen embossed. Now if the space
alluded to between the nucleus and the impression, instead of being left empty,
has been filled up with calcareous spar, flint, pyrites, or other mineral, we
then obtain from the mould an exact cast both of the external and internal form
of the original shell. In this manner silicified casts of shells have been
formed; and if the mud or sand of the nucleus happen to be incoherent, or
soluble in acid, we can then procure in flint an empty shell, which in shape is
the exact counterpart of the original. This cast may be compared to a bronze
statue, representing merely the superficial form, and not the internal
organisation; but there is another description of petrifaction by no means
uncommon, and of a much more wonderful kind, which may be compared to certain
anatomical models in wax, where not only the outward forms and features, but the
nerves, blood-vessels, and other internal organs are also shown. Thus we find
corals, originally calcareous, in which not only the general shape, but also the
minute and complicated internal organisation is retained in flint.
(FIGURE 53. Section of a tree from the coal-measures, magnified (Witham),
showing texture of wood.)
Such a process of petrifaction is still more remarkably exhibited in fossil
wood, in which we often perceive not only the rings of annual growth, but all
the minute vessels and medullary rays. Many of the minute cells and fibres of
plants, and even those spiral vessels which in the living vegetable can only be
discovered by the microscope, are preserved. Among many instances, I may mention
a fossil tree, seventy-two feet in length, found at Gosforth, near Newcastle, in
sandstone strata associated with coal. By cutting a transverse slice so thin as
to transmit light, and magnifying it about fifty-five times, the texture, as
seen in Figure 53, is exhibited. A texture equally minute and complicated has
been observed in the wood of large trunks of fossil trees found in the
Craigleith quarry near Edinburgh, where the stone was not in the slightest
degree siliceous, but consisted chiefly of carbonate of lime, with oxide of
iron, alumina, and carbon. The parallel rows of vessels here seen are the rings
of annual growth, but in one part they are imperfectly preserved, the wood
having probably decayed before the mineralising matter had penetrated to that
portion of the tree.
In attempting to explain the process of petrifaction in such cases, we may first
assume that strata are very generally permeated by water charged with minute
portions of calcareous, siliceous, and other earths in solution. In what manner
they become so impregnated will be afterwards considered. If an organic
substance is exposed in the open air to the action of the sun and rain, it will
in time putrefy, or be dissolved into its component elements, consisting usually
of oxygen, hydrogen, nitrogen, and carbon. These will readily be absorbed by the
atmosphere or be washed away by rain, so that all vestiges of the dead animal or
plant disappear. But if the same substances be submerged in water, they
decompose more gradually; and if buried in earth, still more slowly; as in the
familiar example of wooden piles or other buried timber. Now, if as fast as each
particle is set free by putrefaction in a fluid or gaseous state, a particle
equally minute of carbonate of lime, flint, or other mineral, is at hand ready
to be precipitated, we may imagine this inorganic matter to take the place just
before left unoccupied by the organic molecule. In this manner a cast of the
interior of certain vessels may first be taken, and afterwards the more solid
walls of the same may decay and suffer a like transmutation. Yet when the whole
is lapidified, it may not form one homogeneous mass of stone or metal. Some of
the original ligneous, osseous, or other organic elements may remain mingled in
certain parts, or the lapidifying substance itself may be differently coloured
at different times, or so crystallised as to reflect light differently, and thus
the texture of the original body may be faithfully exhibited.
The student may perhaps ask whether, on chemical principles, we have any ground
to expect that mineral matter will be thrown down precisely in those spots where
organic decomposition is in progress? The following curious experiments may
serve to illustrate this point: Professor Goppert of Breslau, with a view of
imitating the natural process of petrifaction, steeped a variety of animal and
vegetable substances in waters, some holding siliceous, others calcareous,
others metallic matter in solution. He found that in the period of a few weeks,
or sometimes even days, the organic bodies thus immersed were mineralised to a
certain extent. Thus, for example, thin vertical slices of deal, taken from the
Scotch fir (Pinus sylvestris), were immersed in a moderately strong solution of
sulphate of iron. When they had been thoroughly soaked in the liquid for several
days they were dried and exposed to a red-heat until the vegetable matter was
burnt up and nothing remained but an oxide of iron, which was found to have
taken the form of the deal so exactly that casts even of the dotted vessels
peculiar to this family of plants were distinctly visible under the microscope.
The late Dr. Turner observes, that when mineral matter is in a "nascent state,"
that is to say, just liberated from a previous state of chemical combination, it
is most ready to unite with other matter, and form a new chemical compound.
Probably the particles or atoms just set free are of extreme minuteness, and
therefore move more freely, and are more ready to obey any impulse of chemical
affinity. Whatever be the cause, it clearly follows, as before stated, that
where organic matter newly imbedded in sediment is decomposing, there will
chemical changes take place most actively.
An analysis was lately made of the water which was flowing off from the rich mud
deposited by the Hooghly River in the Delta of the Ganges after the annual
inundation. This water was found to be highly charged with carbonic acid holding
lime in solution. (Piddington Asiatic Researches volume 18 page 226.) Now if
newly-deposited mud is thus proved to be permeated by mineral matter in a state
of solution, it is not difficult to perceive that decomposing organic bodies,
naturally imbedded in sediment, may as readily become petrified as the
substances artificially immersed by Professor Goppert in various fluid mixtures.
It is well known that the waters of all springs are more or less charged with
earthy, alkaline, or metallic ingredients derived from the rocks and mineral
veins through which they percolate. Silex is especially abundant in hot springs,
and carbonate of lime is almost always present in greater or less quantity. The
materials for the petrifaction of organic remains are, therefore, usually at
hand in a state of chemical solution wherever organic remains are imbedded in
ELEVATION OF STRATA ABOVE THE SEA.-- HORIZONTAL AND INCLINED STRATIFICATION.
Why the Position of Marine Strata, above the Level of the Sea, should be
referred to the rising up of the Land, not to the going down of the Sea.
Strata of Deep-sea and Shallow-water Origin alternate.
Also Marine and Fresh-water Beds and old Land Surfaces.
Vertical, inclined, and folded Strata.
Anticlinal and Synclinal Curves.
Theories to explain Lateral Movements.
Creeps in Coal-mines.
Dip and Strike.
Structure of the Jura.
Various Forms of Outcrop.
Synclinal Strata forming Ridges.
Connection of Fracture and Flexure of Rocks.
Superficial Signs of the same obliterated by Denudation.
Great Faults the Result of repeated Movements.
Arrangement and Direction of parallel Folds of Strata.
LAND HAS BEEN RAISED, NOT THE SEA LOWERED.
It has been already stated that the aqueous rocks containing marine fossils
extend over wide continental tracts, and are seen in mountain chains rising to
great heights above the level of the sea (Chapter 1). Hence it follows, that
what is now dry land was once under water. But if we admit this conclusion, we
must imagine, either that there has been a general lowering of the waters of the
ocean, or that the solid rocks, once covered by water, have been raised up
bodily out of the sea, and have thus become dry land. The earlier geologists,
finding themselves reduced to this alternative, embraced the former opinion,
assuming that the ocean was originally universal, and had gradually sunk down to
its actual level, so that the present islands and continents were left dry. It
seemed to them far easier to conceive that the water had gone down, than that
solid land had risen upward into its present position. It was, however,
impossible to invent any satisfactory hypothesis to explain the disappearance of
so enormous a body of water throughout the globe, it being necessary to infer
that the ocean had once stood at whatever height marine shells might be
detected. It moreover appeared clear, as the science of geology advanced, that
certain spaces on the globe had been alternately sea, then land, then estuary,
then sea again, and, lastly, once more habitable land, having remained in each
of these states for considerable periods. In order to account for such phenomena
without admitting any movement of the land itself, we are required to imagine
several retreats and returns of the ocean; and even then our theory applies
merely to cases where the marine strata composing the dry land are horizontal,
leaving unexplained those more common instances where strata are inclined,
curved, or placed on their edges, and evidently not in the position in which
they were first deposited.
Geologists, therefore, were at last compelled to have recourse to the doctrine
that the solid land has been repeatedly moved upward or downward, so as
permanently to change its position relatively to the sea. There are several
distinct grounds for preferring this conclusion. First, it will account equally
for the position of those elevated masses of marine origin in which the
stratification remains horizontal, and for those in which the strata are
disturbed, broken, inclined, or vertical. Secondly, it is consistent with human
experience that land should rise gradually in some places and be depressed in
others. Such changes have actually occurred in our own days, and are now in
progress, having been accompanied in some cases by violent convulsions, while in
others they have proceeded so insensibly as to have been ascertainable only by
the most careful scientific observations, made at considerable intervals of
time. On the other hand, there is no evidence from human experience of a rising
or lowering of the sea's level in any region, and the ocean can not be raised or
depressed in one place without its level being changed all over the globe.
These preliminary remarks will prepare the reader to understand the great
theoretical interest attached to all facts connected with the position of
strata, whether horizontal or inclined, curved or vertical.
Now the first and most simple appearance is where strata of marine origin occur
above the level of the sea in horizontal position. Such are the strata which we
meet with in the south of Sicily, filled with shells for the most part of the
same species as those now living in the Mediterranean. Some of these rocks rise
to the height of more than 2000 feet above the sea. Other mountain masses might
be mentioned, composed of horizontal strata of high antiquity, which contain
fossil remains of animals wholly dissimilar from any now known to exist. In the
south of Sweden, for example, near Lake Wener, the beds of some of the oldest
fossiliferous deposits, called Silurian and Cambrian by geologists, occur in as
level a position as if they had recently formed part of the delta of a great
river, and been left dry on the retiring of the annual floods. Aqueous rocks of
equal antiquity extend for hundreds of miles over the lake-district of North
America, and exhibit in like manner a stratification nearly undisturbed. The
Table Mountain at the Cape of Good Hope is another example of highly elevated
yet perfectly horizontal strata, no less than 3500 feet in thickness, and
consisting of sandstone of very ancient date.
Instead of imagining that such fossiliferous rocks were always at their present
level, and that the sea was once high enough to cover them, we suppose them to
have constituted the ancient bed of the ocean, and to have been afterwards
uplifted to their present height. This idea, however startling it may at first
appear, is quite in accordance, as before stated, with the analogy of changes
now going on in certain regions of the globe. Thus, in parts of Sweden, and the
shores and islands of the Gulf of Bothnia, proofs have been obtained that the
land is experiencing, and has experienced for centuries, a slow upheaving
movement. (See "Principles of Geology" 1867 page 314.)
It appears from the observations of Mr. Darwin and others, that very extensive
regions of the continent of South America have been undergoing slow and gradual
upheaval, by which the level plains of Patagonia, covered with recent marine
shells, and the Pampas of Buenos Ayres, have been raised above the level of the
sea. On the other hand, the gradual sinking of the west coast of Greenland, for
the space of more than 600 miles from north to south, during the last four
centuries, has been established by the observations of a Danish naturalist, Dr.
Pingel. And while these proofs of continental elevation and subsidence, by slow
and insensible movements, have been recently brought to light, the evidence has
been daily strengthened of continued changes of level effected by violent
convulsions in countries where earthquakes are frequent. There the rocks are
rent from time to time, and heaved up or thrown down several feet at once, and
disturbed in such a manner as to show how entirely the original position of
strata may be modified in the course of centuries.
Mr. Darwin has also inferred that, in those seas where circular coral islands
and barrier reefs abound, there is a slow and continued sinking of the submarine
mountains on which the masses of coral are based; while there are other areas of
the South Sea where the land is on the rise, and where coral has been upheaved
far above the sea-level.
ALTERNATIONS OF MARINE AND FRESH-WATER STRATA.
It has been shown in the third chapter that there is such a difference between
land, fresh-water, and marine fossils as to enable the geologist to determine
whether particular groups of strata were formed at the bottom of the ocean or in
estuaries, rivers, or lakes. If surprise was at first created by the discovery
of marine corals and shells at the height of several miles above the sea-level,
the imagination was afterwards not less startled by observing that in the
successive strata composing the earth's crust, especially if their total
thickness amounted to thousands of feet, they comprised in some parts formations
of shallow-sea as well as of deep-sea origin; also beds of brackish or even of
purely fresh-water formation, as well as vegetable matter or coal accumulated on
ancient land. In these cases we as frequently find fresh-water beds below a
marine set or shallow-water under those of deep-sea origin as the reverse. Thus,
if we bore an artesian well below London, we pass through a marine clay, and
there reach, at the depth of several hundred feet, a shallow-water and
fluviatile sand, beneath which comes the white chalk originally formed in a deep
sea. Or if we bore vertically through the chalk of the North Downs, we come,
after traversing marine chalky strata, upon a fresh-water formation many
hundreds of feet thick, called the Wealden, such as is seen in Kent and Surrey,
which is known in its turn to rest on purely marine beds. In like manner, in
various parts of Great Britain we sink vertical shafts through marine deposits
of great thickness, and come upon coal which was formed by the growth of plants
on an ancient land-surface sometimes hundreds of square miles in extent.
VERTICAL, INCLINED, AND CURVED STRATA.
(FIGURE 54. Vertical conglomerate and sandstone.)
It has been stated that marine strata of different ages are sometimes found at a
considerable height above the sea, yet retaining their original horizontality;
but this state of things is quite exceptional. As a general rule, strata are
inclined or bent in such a manner as to imply that their original position has
(FIGURE 55. Section of Forfarshire, from N.W. to S.E., from the foot of the
Grampians to the sea at Arbroath (volcanic or trap rocks omitted). Length of
section twenty miles.
From S.E. (left) Sea: Whiteness, Arbroath: Strata a, 2, 3: Leys Mill: Strata 4:
Sidlaw Hills. Viney R.: Strata B: Pitmuies: Strata 4: Position and nature of the
rocks below No. 4 unknown: Turin: Findhaven: Strata 3, 2, A: Valley of
Strathmore: Strata 1, 2, 3: W. Ogle: Strata 4 and Clay-Slate: to N.W. (right).)
The most unequivocal evidence of such a change is afforded by their standing up
vertically, showing their edges, which is by no means a rare phenomenon,
especially in mountainous countries. Thus we find in Scotland, on the southern
skirts of the Grampians, beds of pudding-stone alternating with thin layers of
fine sand, all placed vertically to the horizon. When Saussure first observed
certain conglomerates in a similar position in the Swiss Alps, he remarked that
the pebbles, being for the most part of an oval shape, had their longer axes
parallel to the planes of stratification (see Figure 54). From this he inferred
that such strata must, at first, have been horizontal, each oval pebble having
settled at the bottom of the water, with its flatter side parallel to the
horizon, for the same reason that an egg will not stand on either end if
unsupported. Some few, indeed, of the rounded stones in a conglomerate
occasionally afford an exception to the above rule, for the same reason that in
a river's bed, or on a shingle beach, some pebbles rest on their ends or edges;
these having been shoved against or between other stones by a wave or current,
so as to assume this position.
ANTICLINAL AND SYNCLINAL CURVES.
Vertical strata, when they can be traced continuously upward or downward for
some depth, are almost invariably seen to be parts of great curves, which may
have a diameter of a few yards, or of several miles. I shall first describe two
curves of considerable regularity, which occur in Forfarshire, extending over a
country twenty miles in breadth, from the foot of the Grampians to the sea near
The mass of strata here shown may be 2000 feet in thickness, consisting of red
and white sandstone, and various coloured shales, the beds being distinguishable
into four principal groups, namely, No. 1, red marl or shale; No. 2, red
sandstone, used for building; No. 3, conglomerate; and No. 4, grey paving-stone,
and tile-stone, with green and reddish shale, containing peculiar organic
remains. A glance at the section (Figure 55.) will show that each of the
formations 2, 3, 4 are repeated thrice at the surface, twice with a southerly,
and once with a northerly inclination or DIP, and the beds in No. 1, which are
nearly horizontal, are still brought up twice by a slight curvature to the
surface, once on each side of A. Beginning at the north-west extremity, the
tile-stones and conglomerates, No. 4 and No. 3, are vertical, and they generally
form a ridge parallel to the southern skirts of the Grampians. The superior
strata, Nos. 2 and 1, become less and less inclined on descending to the valley
of Strathmore, where the strata, having a concave bend, are said by geologists
to lie in a "trough" or "basin." Through the centre of this valley runs an
imaginary line A, called technically a "synclinal line," where the beds, which
are tilted in opposite directions, may be supposed to meet. It is most important
for the observer to mark such lines, for he will perceive by the diagram that,
in travelling from the north to the centre of the basin, he is always passing
from older to newer beds; whereas, after crossing the line A, and pursuing his
course in the same southerly direction, he is continually leaving the newer, and
advancing upon older strata. All the deposits which he had before examined begin
then to recur in reversed order, until he arrives at the central axis of the
Sidlaw hills, where the strata are seen to form an arch, or SADDLE, having an
ANTICLINAL line, B, in the centre. On passing this line, and continuing towards
the S.E., the formations 4, 3, and 2, are again repeated, in the same relative
order of superposition, but with a southerly dip. At Whiteness (see Figure 55)
it will be seen that the inclined strata are covered by a newer deposit, a, in
horizontal beds. These are composed of red conglomerate and sand, and are newer
than any of the groups, 1, 2, 3, 4, before described, and rest UNCONFORMABLY
upon strata of the sandstone group, No. 2.
An example of curved strata, in which the bends or convolutions of the rock are
sharper and far more numerous within an equal space, has been well described by
Sir James Hall. (Edinburgh Transactions volume 7 plate 3.) It occurs near St.
Abb's Head, on the east coast of Scotland, where the rocks consist principally
of a bluish slate, having frequently a ripple-marked surface. The undulations of
the beds reach from the top to the bottom of cliffs from 200 to 300 feet in
height, and there are sixteen distinct bendings in the course of about six
miles, the curvatures being alternately concave and convex upward.
FOLDING BY LATERAL MOVEMENT.
(FIGURE 56. Curved strata of slate near St. Abb's Head, Berwickshire. (Sir J.
(FIGURE 57. Curved strata in line of cliff.)
(FIGURE 58. Folded cloths imitating bent strata.)
An experiment was made by Sir James Hall, with a view of illustrating the manner
in which such strata, assuming them to have been originally horizontal, may have
been forced into their present position. A set of layers of clay were placed
under a weight, and their opposite ends pressed towards each other with such
force as to cause them to approach more nearly together. On the removal of the
weight, the layers of clay were found to be curved and folded, so as to bear a
miniature resemblance to the strata in the cliffs. We must, however, bear in
mind that in the natural section or sea-cliff we only see the foldings
imperfectly, one part being invisible beneath the sea, and the other, or upper
portion, being supposed to have been carried away by DENUDATION, or that action
of water which will be explained in the next chapter. The dark lines in the plan
(Figure 57) represent what is actually seen of the strata in the line of cliff
alluded to; the fainter lines, that portion which is concealed beneath the sea-
level, as also that which is supposed to have once existed above the present
We may still more easily illustrate the effects which a lateral thrust might
produce on flexible strata, by placing several pieces of differently coloured
cloths upon a table, and when they are spread out horizontally, cover them with
a book. Then apply other books to each end, and force them towards each other.
The folding of the cloths (see Figure 58) will imitate those of the bent strata;
the incumbent book being slightly lifted up, and no longer touching the two
volumes on which it rested before, because it is supported by the tops of the
anticlinal ridges formed by the curved cloths. In like manner there can be no
doubt that the squeezed strata, although laterally condensed and more closely
packed, are yet elongated and made to rise upward, in a direction perpendicular
to the pressure.
Whether the analogous flexures in stratified rocks have really been due to
similar sideway movements is a question which we can not decide by reference to
our own observation. Our inability to explain the nature of the process is,
perhaps, not simply owing to the inaccessibility of the subterranean regions
where the mechanical force is exerted, but to the extreme slowness of the
movement. The changes may sometimes be due to variation in the temperature of
mountain masses of rock causing them, while still solid, to expand or contract;
or melting them, and then again cooling them and allowing them to crystallise.
If such be the case, we have scarcely more reason to expect to witness the
operation of the process within the limited periods of our scientific
observation than to see the swelling of the roots of a tree, by which, in the
course of years, a wall of solid masonry may be lifted up, rent or thrown down.
In both instances the force may be irresistible, but though adequate, it need
not be visible by us, provided the time required for its development be very
great. The lateral pressure arising from the unequal expansion of rocks by heat
may cause one mass lying in the same horizontal plane gradually to occupy a
larger space, so as to press upon another rock, which, if flexible, may be
squeezed into a bent and folded form. It will also appear, when the volcanic and
granitic rocks are described, that some of them have, when melted in the
interior of the earth's crust, been injected forcibly into fissures, and after
the solidification of such intruded matter, other sets of rents, crossing the
first, have been formed and in their turn filled by melted rock. Such repeated
injections imply a stretching, and often upheaval, of the whole mass.
We also know, especially by the study of regions liable to earthquakes, that
there are causes at work in the interior of the earth capable of producing a
sinking in of the ground, sometimes very local, but often extending over a wide
area. The continuance of such a downward movement, especially if partial and
confined to linear areas, may produce regular folds in the strata.
CREEPS IN COAL-MINES.
The "creeps," as they are called in coal-mines, afford an excellent illustration
of this fact.-- First, it may be stated generally, that the excavation of coal
at a considerable depth causes the mass of overlying strata to sink down bodily,
even when props are left to support the roof of the mine. "In Yorkshire," says
Mr. Buddle, "three distinct subsidences were perceptible at the surface, after
the clearing out of three seams of coal below, and innumerable vertical cracks
were caused in the incumbent mass of sandstone and shale which thus settled
down." (Proceedings of Geological Society volume 3 page 148.) The exact amount
of depression in these cases can only be accurately measured where water
accumulates on the surface, or a railway traverses a coal-field.
(FIGURE 59. Section of carboniferous strata at Wallsend, Newcastle, showing
"creeps." (J. Buddle, Esq.)
Horizontal length of section 174 feet. The upper seam, or main coal, here worked
out, was 630 feet below the surface.
Section through, from top to bottom:
1. Main coal, 6 feet 6 inches, with creeps a, b, c, d.
Shale eighteen yards thick.
2. Metal coal, 3 feet, with fractures e, f, g, h.)
When a bed of coal is worked out, pillars or rectangular masses of coal are left
at intervals as props to support the roof, and protect the colliers. Thus in
Figure 59, representing a section at Wallsend, Newcastle, the galleries which
have been excavated are represented by the white spaces a, b, while the
adjoining dark portions are parts of the original coal seam left as props, beds
of sandy clay or shale constituting the floor of the mine. When the props have
been reduced in size, they are pressed down by the weight of overlying rocks (no
less than 630 feet thick) upon the shale below, which is thereby squeezed and
forced up into the open spaces.
Now it might have been expected that, instead of the floor rising up, the
ceiling would sink down, and this effect, called a "thrust," does, in fact, take
place where the pavement is more solid than the roof. But it usually happens, in
coalmines, that the roof is composed of hard shale, or occasionally of
sandstone, more unyielding than the foundation, which often consists of clay.
Even where the argillaceous substrata are hard at first, they soon become
softened and reduced to a plastic state when exposed to the contact of air and
water in the floor of a mine.
The first symptom of a "creep," says Mr. Buddle, is a slight curvature at the
bottom of each gallery, as at a, Figure 59: then the pavement, continuing to
rise, begins to open with a longitudinal crack, as at b; then the points of the
fractured ridge reach the roof, as at c; and, lastly, the upraised beds close up
the whole gallery, and the broken portions of the ridge are reunited and
flattened at the top, exhibiting the flexure seen at d. Meanwhile the coal in
the props has become crushed and cracked by pressure. It is also found that
below the creeps a, b, c, d, an inferior stratum, called the "metal coal," which
is 3 feet thick, has been fractured at the points e, f, g, h, and has risen, so
as to prove that the upward movement, caused by the working out of the "main
coal," has been propagated through a thickness of 54 feet of argillaceous beds,
which intervene between the two coal-seams. This same displacement has also been
traced downward more than 150 feet below the metal coal, but it grows
continually less and less until it becomes imperceptible.
No part of the process above described is more deserving of our notice than the
slowness with which the change in the arrangement of the beds is brought about.
Days, months, or even years, will sometimes elapse between the first bending of
the pavement and the time of its reaching the roof. Where the movement has been
most rapid, the curvature of the beds is most regular, and the reunion of the
fractured ends most complete; whereas the signs of displacement or violence are
greatest in those creeps which have required months or years for their entire
accomplishment. Hence we may conclude that similar changes may have been wrought
on a larger scale in the earth's crust by partial and gradual subsidences,
especially where the ground has been undermined throughout long periods of time;
and we must be on our guard against inferring sudden violence, simply because
the distortion of the beds is excessive.
Engineers are familiar with the fact that when they raise the level of a railway
by heaping stone or gravel on a foundation of marsh, quicksand, or other
yielding formation, the new mound often sinks for a time as fast as they attempt
to elevate it; when they have persevered so as to overcome this difficulty, they
frequently find that some of the adjoining flexible ground has risen up in one
or more parallel arches or folds, showing that the vertical pressure of the
sinking materials has given rise to a lateral folding movement.
In like manner, in the interior of the earth, the solid parts of the earth's
crust may sometimes, as before mentioned, be made to expand by heat, or may be
pressed by the force of steam against flexible strata loaded with a great weight
of incumbent rocks. In this case the yielding mass, squeezed, but unable to
overcome the resistance which it meets with in a vertical direction, may be
gradually relieved by lateral folding.
DIP AND STRIKE.
(FIGURE 60. Series of inclined strata dipping to the north at an angle of 45
In describing the manner in which strata depart from their original
horizontality, some technical terms, such as "dip" and "strike," "anticlinal"
and "synclinal" line or axis, are used by geologists. I shall now proceed to
explain some of these to the student. If a stratum or bed of rock, instead of
being quite level, be inclined to one side, it is said to DIP; the point of the
compass to which it is inclined is called the POINT OF DIP, and the degree of
deviation from a level or horizontal line is called THE AMOUNT OF DIP, or THE
ANGLE OF DIP. Thus, in the diagram (Figure 60), a series of strata are inclined,
and they dip to the north at an angle of forty-five degrees. The STRIKE, or LINE
OF BEARING, is the prolongation or extension of the strata in a direction AT
RIGHT ANGLES to the dip; and hence it is sometimes called the DIRECTION of the
strata. Thus, in the above instance of strata dipping to the north, their strike
must necessarily be east and west. We have borrowed the word from the German
geologists, streichen signifying to extend, to have a certain direction. Dip and
strike may be aptly illustrated by a row of houses running east and west, the
long ridge of the roof representing the strike of the stratum of slates, which
dip on one side to the north, and on the other to the south.
A stratum which is horizontal, or quite level in all directions, has neither dip
It is always important for the geologist, who is endeavouring to comprehend the
structure of a country, to learn how the beds dip in every part of the district;
but it requires some practice to avoid being occasionally deceived, both as to
the point of dip and the amount of it.
(FIGURE 61. Apparent horizontality of inclined strata.)
If the upper surface of a hard stony stratum be uncovered, whether artificially
in a quarry, or by waves at the foot of a cliff, it is easy to determine towards
what point of the compass the slope is steepest, or in what direction water
would flow if poured upon it. This is the true dip. But the edges of highly
inclined strata may give rise to perfectly horizontal lines in the face of a
vertical cliff, if the observer see the strata in the line of the strike, the
dip being inward from the face of the cliff. If, however, we come to a break in
the cliff, which exhibits a section exactly at right angles to the line of the
strike, we are then able to ascertain the true dip. In the drawing (Figure 61),
we may suppose a headland, one side of which faces to the north, where the beds
would appear perfectly horizontal to a person in the boat; while in the other
side facing the west, the true dip would be seen by the person on shore to be at
an angle of 40 degrees. If, therefore, our observations are confined to a
vertical precipice facing in one direction, we must endeavour to find a ledge or
portion of the plane of one of the beds projecting beyond the others, in order
to ascertain the true dip.
(FIGURE 62. Two hands used to determine the inclination of strata.)
If not provided with a clinometer, a most useful instrument, when it is of
consequence to determine with precision the inclination of the strata, the
observer may measure the angle within a few degrees by standing exactly opposite
to a cliff where the true dip is exhibited, holding the hands immediately before
the eyes, and placing the fingers of one in a perpendicular, and of the other in
a horizontal position, as in Figure 62. It is thus easy to discover whether the
lines of the inclined beds bisect the angle of 90 degrees, formed by the meeting
of the hands, so as to give an angle of 45 degrees, or whether it would divide
the space into two equal or unequal portions. You have only to change hands to
get the line of dip on the upper side of the horizontal hand.
(FIGURE 63. Section illustrating the structure of the Swiss Jura.)
It has been already seen, in describing the curved strata on the east coast of
Scotland, in Forfarshire and Berwickshire, that a series of concave and convex
bendings are occasionally repeated several times. These usually form part of a
series of parallel waves of strata, which are prolonged in the same direction,
throughout a considerable extent of country. Thus, for example, in the Swiss
Jura, that lofty chain of mountains has been proved to consist of many parallel
ridges, with intervening longitudinal valleys, as in Figure 63, the ridges being
formed by curved fossiliferous strata, of which the nature and dip are
occasionally displayed in deep transverse gorges, called "cluses," caused by
fractures at right angles to the direction of the chain. (Thurmann "Essai sur
les Soulevemens Jurassiques de Porrentruy" Paris 1832.) Now let us suppose these
ridges and parallel valleys to run north and south, we should then say that the
STRIKE of the beds is north and south, and the DIP east and west. Lines drawn
along the summits of the ridges, A, B, would be anticlinal lines, and one
following the bottom of the adjoining valleys a synclinal line.
OUTCROP OF STRATA.
(FIGURE 64. Ground-plan of the denuded ridge C, Figure 63.)
(FIGURE 65. Transverse section of the denuded ridge C, Figure 63..)
It will be observed that some of these ridges, A, B, are unbroken on the summit,
whereas one of them, C, has been fractured along the line of strike, and a
portion of it carried away by denudation, so that the ridges of the beds in the
formations a, b, c come out to the day, or, as the miners say, CROP OUT, on the
sides of a valley. The ground-plan of such a denuded ridge as C, as given in a
geological map, may be expressed by the diagram, Figure 64, and the cross-
section of the same by Figure 65. The line D E, Figure 64, is the anticlinal
line, on each side of which the dip is in opposite directions, as expressed by
the arrows. The emergence of strata at the surface is called by miners their
OUTCROP, or BASSET.
If, instead of being folded into parallel ridges, the beds form a boss or dome-
shaped protuberance, and if we suppose the summit of the dome carried off, the
ground-plan would exhibit the edges of the strata forming a succession of
circles, or ellipses, round a common centre. These circles are the lines of
strike, and the dip being always at right angles is inclined in the course of
the circuit to every point of the compass, constituting what is termed a qua-
quaversal dip-- that is, turning every way.
There are endless variations in the figures described by the basset-edges of the
strata, according to the different inclination of the beds, and the mode in
which they happen to have been denuded. One of the simplest rules, with which
every geologist should be acquainted, relates to the V-like form of the beds as
they crop out in an ordinary valley. First, if the strata be horizontal, the V-
like form will be also on a level, and the newest strata will appear at the
(FIGURE 66. Slope of valley 40 degrees, dip of strata 20 degrees.)
Secondly, if the beds be inclined and intersected by a valley sloping in the
same direction, and the dip of the beds be less steep than the slope of the
valley, then the V's, as they are often termed by miners, will point upward (see
Figure 66), those formed by the newer beds appearing in a superior position, and
extending highest up the valley, as A is seen above B.
(FIGURE 67. Slope of valley 20 degrees, dip of strata 50 degrees.)
Thirdly, if the dip of the beds be steeper than the slope of the valley, then
the V's will point downward (see Figure 67), and those formed of the older beds
will now appear uppermost, as B appears above A.
(FIGURE 68. Slope of valley 20 degrees, dip of strata 20 degrees, in opposite
Fourthly, in every case where the strata dip in a contrary direction to the
slope of the valley, whatever be the angle of inclination, the newer beds will
appear the highest, as in the first and second cases. This is shown by the
drawing (Figure 68), which exhibits strata rising at an angle of 20 degrees, and
crossed by a valley, which declines in an opposite direction at 20 degrees.
These rules may often be of great practical utility; for the different degrees
of dip occurring in the two cases represented in Figures 66 and 67 may
occasionally be encountered in following the same line of flexure at points a
few miles distant from each other. A miner unacquainted with the rule, who had
first explored the valley Figure 66, may have sunk a vertical shaft below the
coal-seam A, until he reached the inferior bed, B. He might then pass to the
valley, Figure 67, and discovering there also the outcrop of two coal-seams,
might begin his workings in the uppermost in the expectation of coming down to
the other bed A, which would be observed cropping out lower down the valley. But
a glance at the section will demonstrate the futility of such hopes. (I am
indebted to the kindness of T. Sopwith, Esq., for three models which I have
copied in the above diagrams; but the beginner may find it by no means easy to
understand such copies, although, if he were to examine and handle the
originals, turning them about in different ways, he would at once comprehend
their meaning, as well as the import of others far more complicated, which the
same engineer has constructed to illustrate FAULTS.)
SYNCLINAL STRATA FORMING RIDGES.
(FIGURE 69. Section of carboniferous rocks of Lancashire. (E. Hull. (Edward
Hull, Quarterly Geological Journal volume 24 page 324. 1868.))
a. Synclinal. Grits and shales.
c. Anticlinal. Mountain limestone.
b. Synclinal. Grits and shales.)
Although in many cases an anticlinal axis forms a ridge, and a synclinal axis a
valley, as in A B, Figure 63, yet this can by no means be laid down as a general
rule, as the beds very often slope inward from either side of a mountain, as at
a, b, Figure 69, while in the intervening valley, c, they slope upward, forming
It would be natural to expect the fracture of solid rocks to take place chiefly
where the bending of the strata has been sharpest, and such rending may produce
ravines giving access to running water and exposing the surface to atmospheric
waste. The entire absence, however, of such cracks at points where the strain
must have been greatest, as at a, Figure 63, is often very remarkable, and not
always easy of explanation. We must imagine that many strata of limestone,
chert, and other rocks which are now brittle, were pliant when bent into their
present position. They may have owed their flexibility in part to the fluid
matter which they contained in their minute pores, as before described, and in
part to the permeation of sea-water while they were yet submerged.
(FIGURE 70. Strata of chert, grit, and marl, near St. Jean de Luz.)
At the western extremity of the Pyrenees, great curvatures of the strata are
seen in the sea-cliffs, where the rocks consist of marl, grit, and chert. At
certain points, as at a, Figure 70, some of the bendings of the flinty chert are
so sharp that specimens might be broken off well fitted to serve as ridge-tiles
on the roof of a house. Although this chert could not have been brittle as now,
when first folded into this shape, it presents, nevertheless, here and there, at
the points of greatest flexure, small cracks, which show that it was solid, and
not wholly incapable of breaking at the period of its displacement. The numerous
rents alluded to are not empty, but filled with chalcedony and quartz.
(FIGURE 71. Bent and undulating gypseous marl.
g. Gypsum. m. Marl.)
Between San Caterina and Castrogiovanni, in Sicily, bent and undulating gypseous
marls occur, with here and there thin beds of solid gypsum interstratified.
Sometimes these solid layers have been broken into detached fragments, still
preserving their sharp edges (g, g, Figure 71), while the continuity of the more
pliable and ductile marls, m, m, has not been interrupted.
(FIGURE 72. Folded strata.)
(FIGURE 73. Folded strata.)
We have already explained, Figure 69, that stratified rocks have usually their
strata bent into parallel folds forming anticlinal and synclinal axes, a group
of several of these folds having often been subjected to a common movement, and
having acquired a uniform strike or direction. In some disturbed regions these
folds have been doubled back upon themselves in such a manner that it is often
difficult for an experienced geologist to determine correctly the relative age
of the beds by superposition. Thus, if we meet with the strata seen in the
section, Figure 72, we should naturally suppose that there were twelve distinct
beds, or sets of beds, No. 1 being the newest, and No. 12 the oldest of the
series. But this section may perhaps exhibit merely six beds, which have been
folded in the manner seen in Figure 73, so that each of them is twice repeated,
the position of one half being reversed, and part of No. 1, originally the
uppermost, having now become the lowest of the series.
These phenomena are observable on a magnificent scale in certain regions in
Switzerland, in precipices often more than 2000 feet in perpendicular height,
and there are flexures not inferior in dimensions in the Pyrenees. The upper
part of the curves seen in this diagram, Figure 73, and expressed in fainter
lines, has been removed by what is called denudation, to be afterwards
FRACTURES OF THE STRATA AND FAULTS.
Numerous rents may often be seen in rocks which appear to have been simply
broken, the fractured parts still remaining in contact; but we often find a
fissure, several inches or yards wide, intervening between the disunited
portions. These fissures are usually filled with fine earth and sand, or with
angular fragments of stone, evidently derived from the fracture of the
The face of each wall of the fissure is often beautifully polished, as if
glazed, striated, or scored with parallel furrows and ridges, such as would be
produced by the continued rubbing together of surfaces of unequal hardness.
These polished surfaces are called by miners "slickensides." It is supposed that
the lines of the striae indicate the direction in which the rocks were moved.
During one of the minor earthquakes in Chili, in 1840, the brick walls of a
building were rent vertically in several places, and made to vibrate for several
minutes during each shock, after which they remained uninjured, and without any
opening, although the line of each crack was still visible. When all movement
had ceased, there were seen on the floor of the house, at the bottom of each
rent, small heaps of fine brick-dust, evidently produced by trituration.
(FIGURE 74. Faults. A B perpendicular, C D oblique to the horizon.)
(FIGURE 75. E F, fault or fissure filled with rubbish, on each side of which the
shifted strata are not parallel.)
It is not uncommon to find the mass of rock on one side of a fissure thrown up
above or down below the mass with which it was once in contact on the other
side. "This mode of displacement is called a fault, shift, slip, or throw." "The
miner," says Playfair, describing a fault, "is often perplexed, in his
subterranean journey, by a derangement in the strata, which changes at once all
those lines and bearings which had hitherto directed his course. When his mine
reaches a certain plane, which is sometimes perpendicular, as in A B, Figure 74,
sometimes oblique to the horizon (as in C D, ibid.), he finds the beds of rock
broken asunder, those on the one side of the plane having changed their place,
by sliding in a particular direction along the face of the others. In this
motion they have sometimes preserved their parallelism, as in Figure 74, so that
the strata on each side of faults A B, C D, continue parallel to one another; in
other cases, the strata on each side are inclined, as in a, b, c, d (Figure 75),
though their identity is still to be recognised by their possessing the same
thickness and the same internal characters." (Playfair, Illustration of Hutt.
Theory paragraph 42.)
In Coalbrook Dale, says Mr. Prestwich (Geological Transactions second series
volume 5 page 452.), deposits of sandstone, shale, and coal, several thousand
feet thick, and occupying an area of many miles, have been shivered into
fragments, and the broken remnants have been placed in very discordant
positions, often at levels differing several hundred feet from each other. The
sides of the faults, when perpendicular, are commonly several yards apart, and
are sometimes as much as 50 yards asunder, the interval being filled with broken
debris of the strata. In following the course of the same fault it is sometimes
found to produce in different places very unequal changes of level, the amount
of shift being in one place 300, and in another 700 feet, which arises from the
union of two or more faults. In other words, the disjointed strata have in
certain districts been subjected to renewed movements, which they have not
We may occasionally see exact counterparts of these slips, on a small scale, in
pits of loose sand and gravel, many of which have doubtless been caused by the
drying and shrinking of argillaceous and other beds, slight subsidences having
taken place from failure of support. Sometimes, however, even these small slips
may have been produced during earthquakes; for land has been moved, and its
level, relatively to the sea, considerably altered, within the period when much
of the alluvial sand and gravel now covering the surface of continents was
I have already stated that a geologist must be on his guard, in a region of
disturbed strata, against inferring repeated alternations of rocks, when, in
fact, the same strata, once continuous, have been bent round so as to recur in
the same section, and with the same dip. A similar mistake has often been
occasioned by a series of faults.
(FIGURE 76. Apparent alternations of strata caused by vertical faults.)
If, for example, the dark line A H (Figure 76) represent the surface of a
country on which the strata a, b, c frequently crop out, an observer who is
proceeding from H to A might at first imagine that at every step he was
approaching new strata, whereas the repetition of the same beds has been caused
by vertical faults, or downthrows. Thus, suppose the original mass, A, B, C, D,
to have been a set of uniformly inclined strata, and that the different masses
under E F, F G, and G D sank down successively, so as to leave vacant the spaces
marked in the diagram by dotted lines, and to occupy those marked by the
continuous lines, then let denudation take place along the line A H, so that the
protruding masses indicated by the fainter lines are swept away-- a miner, who
has not discovered the faults, finding the mass a, which we will suppose to be a
bed of coal four times repeated, might hope to find four beds, workable to an
indefinite depth, but first, on arriving at the fault G, he is stopped suddenly
in his workings, for he comes partly upon the shale b, and partly on the
sandstone c; the same result awaits him at the fault F, and on reaching E he is
again stopped by a wall composed of the rock d.
The very different levels at which the separated parts of the same strata are
found on the different sides of the fissure, in some faults, is truly
astonishing. One of the most celebrated in England is that called the "ninety-
fathom dike," in the coal-field of Newcastle. This name has been given to it,
because the same beds are ninety fathoms (540 feet) lower on the northern than
they are on the southern side. The fissure has been filled by a body of sand,
which is now in the state of sandstone, and is called the dike, which is
sometimes very narrow, but in other places more than twenty yards wide.
(Conybeare and Phillips Outlines, etc. page 376.) The walls of the fissure are
scored by grooves, such as would have been produced if the broken ends of the
rock had been rubbed along the plane of the fault. (Phillips Geology Lardner's
Cyclop. page 41.) In the Tynedale and Craven faults, in the north of England,
the vertical displacement is still greater, and the fracture has extended in a
horizontal direction for a distance of thirty miles or more.
GREAT FAULTS THE RESULT OF REPEATED MOVEMENTS.
It must not, however, be supposed that faults generally consist of single linear
rents; there are usually a number of faults springing off from the main one, and
sometimes a long strip of country seems broken up into fragments by sets of
parallel and connecting transverse faults. Oftentimes a great line of fault has
been repeated, or the movements have been continued through successive periods,
so that, newer deposits having covered the old line of displacement, the strata
both newer and older have given way along the old line of fracture. Some
geologists have considered it necessary to imagine that the upward or downward
movement in these cases was accomplished at a single stroke, and not by a series
of sudden but interrupted movements. They appear to have derived this idea from
a notion that the grooved walls have merely been rubbed in one direction, which
is far from being a constant phenomenon. Not only are some sets of striae not
parallel to others, but the clay and rubbish between the walls, when squeezed or
rubbed, have been streaked in different directions, the grooves which the harder
minerals have impressed on the softer being frequently curved and irregular.
(FIGURE 77. Faults and denuded coal-strata, Ashby de la Zouch. (Mammatt.))
The usual absence of protruding masses of rock forming precipices or ridges
along the lines of great faults has already been alluded to in explaining Figure
76, and the same remarkable fact is well exemplified in every coal-field which
has been extensively worked. It is in such districts that the former relation of
the beds which have been shifted is determinable with great accuracy. Thus in
the coal-field of Ashby de la Zouch, in Leicestershire (see Figure 77), a fault
occurs, on one side of which the coal-beds a, b, c, d must once have risen to
the height of 500 feet above the corresponding beds on the other side. But the
uplifted strata do not stand up 500 feet above the general surface; on the
contrary, the outline of the country, as expressed by the line z z, is uniformly
undulating, without any break, and the mass indicated by the dotted outline must
have been washed away. (See Mammatt's Geological Facts etc. page 90 and plate.)
The student may refer to Mr. Hull's measurement of faults, observed in the
Lancashire coal-field, where the vertical displacement has amounted to thousands
of feet, and yet where all the superficial inequalities which must have resulted
from such movements have been obliterated by subsequent denudation. In the same
memoir proofs are afforded of there having been two periods of vertical movement
in the same fault-- one, for example, before, and another after, the Triassic
epoch. (Hull Quarterly Geological Journal volume 24 page 318. 1868.)
The shifting of the beds by faults is often intimately connected with those same
foldings which constitute the anticlinal and synclinal axes before alluded to,
and there is no doubt that the subterranean causes of both forms of disturbance
are to a great extent the same. A fault in Virginia, believed to imply a
displacement of several thousand feet, has been traced for more than eighty
miles in the same direction as the foldings of the Appalachian chain. (H.D.
Rogers Geology of Pennsylvania page 897.) An hypothesis which attributes such a
change of position to a succession of movements, is far preferable to any theory
which assumes each fault to have been accomplished by a single upcast or
downthrow of several thousand feet. For we know that there are operations now in
progress, at great depths in the interior of the earth, by which both large and
small tracts of ground are made to rise above and sink below their former level,
some slowly and insensibly, others suddenly and by starts, a few feet or yards
at a time; whereas there are no grounds for believing that, during the last 3000
years at least, any regions have been either upheaved or depressed, at a single
stroke, to the amount of several hundred, much less several thousand feet.
It is certainly not easy to understand how in the subterranean regions one mass
of solid rock should have been folded up by a continued series of movements,
while another mass in contact, or only separated by a line of fissure, has
remained stationary or has perhaps subsided. But every volcano, by the
intermittent action of the steam, gases, and lava evolved during an eruption,
helps us to form some idea of the manner in which such operations take place.
For eruptions are repeated at uncertain intervals throughout the whole or a
large part of a geological period, some of the surrounding and contiguous
districts remaining quite undisturbed. And in most of the instances with which
we are best acquainted the emission of lava, scoria, and steam is accompanied by
the uplifting of the solid crust. Thus in Vesuvius, Etna, the Madeiras, the
Canary Islands, and the Azores there is evidence of marine deposits of recent
and tertiary date having been elevated to the height of a thousand feet, and
sometimes more, since the commencement of the volcanic explosions. There is,
moreover, a general tendency in contemporaneous volcanic vents to affect a
linear arrangement, extending in some instances, as in the Andes or the Indian
Archipelago, to distances equalling half the circumference of the globe. Where
volcanic heat, therefore, operates at such a depth as not to obtain vent at the
surface, in the form of an eruption, it may nevertheless be conceived to give
rise to upheavals, foldings, and faults in certain linear tracts. And marine
denudation, to be treated of in the next chapter, will help us to understand why
that which should be the protruding portion of the faulted rocks is missing at
ARRANGEMENT AND DIRECTION OF PARALLEL FOLDS OF STRATA.
The possible causes of the folding of strata by lateral movements have been
considered in a former part of this chapter. No European chain of mountains
affords so remarkable an illustration of the persistency of such flexures for a
great distance as the Appalachians before alluded to, and none has been studied
and described by many good observers with more accuracy. The chain extends from
north to south, or rather N.N.E. to S.S.W., for nearly 1500 miles, with a
breadth of 50 miles, throughout which the Palaeozoic strata have been so bent as
to form a series of parallel anticlinal and synclinal ridges and troughs,
comprising usually three or four principal and many smaller plications, some of
them forming broad and gentle arches, others narrower and steeper ones, while
some, where the bending has been greatest, have the position of their beds
inverted, as before shown in Figure 73.
The strike of the parallel ridges, after continuing in a straight line for many
hundred miles, is then found to vary for a more limited distance as much as 30
degrees, the folds wheeling round together in the new direction and continuing
to be parallel, as if they had all obeyed the same movement. The date of the
movements by which the great flexures were brought about must, of course, be
subsequent to the formation of the uppermost part of the coal or the newest of
the bent rocks, but the disturbance must have ceased before the Triassic strata
were deposited on the denuded edges of the folded beds.
The manner in which the numerous parallel folds, all simultaneously formed,
assume a new direction common to the whole of them, and sometimes varying at an
angle of 30 degrees from the normal strike of the chain, shows what deviation
from an otherwise uniform strike of the beds may be experienced when the
geographical area through which they are traced is on so vast a scale.
The disturbances in the case here adverted to occurred between the Carboniferous
period and that of the Trias, and this interval is so vast that they may have
occupied a great lapse of time, during which their parallelism was always
preserved. But, as a rule, wherever after a long geological interval the
recurrence of lateral movements gives rise to a new set of folds, the strike of
these last is different. Thus, for example, Mr. Hull has pointed out that three
principal lines of disturbance, all later than the Carboniferous period, have
affected the stratified rocks of Lancashire. The first of these, having an
E.N.E. direction, took place at the close of the Carboniferous period. The next,
running north and south, at the close of the Permian, and the third, having a
N.N.W. direction, at the close of the Jurassic period. (Edward Hull Quarterly
Geological Journal volume 24 page 323.)
UNCONFORMABILITY OF STRATA.
(FIGURE 78. Unconformable junction of old red sandstone and Silurian schist at
the Siccar Point, near St. Abb's Head, Berwickshire.)
Strata are said to be unconformable when one series is so placed over another
that the planes of the superior repose on the edges of the inferior (see Figure
78.) In this case it is evident that a period had elapsed between the production
of the two sets of strata, and that, during this interval, the older series had
been tilted and disturbed. Afterwards the upper series was thrown down in
horizontal strata upon it. If these superior beds, d d Figure 78, are also
inclined, it is plain that the lower strata a a, have been twice displaced;
first, before the deposition of the newer beds, d d, and a second time when
these same strata were upraised out of the sea, and thrown slightly out of the
(FIGURE 79. Junction of unconformable strata near Mons, in Belgium.)
It often happens that in the interval between the deposition of two sets of
unconformable strata, the inferior rock has not only been denuded, but drilled
by perforating shells. Thus, for example, at Autreppe and Gusigny, near Mons,
beds of an ancient (primary or palaeozoic) limestone, highly inclined, and often
bent, are covered with horizontal strata of greenish and whitish marls of the
Cretaceous formation. The lowest, and therefore the oldest, bed of the
horizontal series is usually the sand and conglomerate, a, in which are rounded
fragments of stone, from an inch to two feet in diameter. These fragments have
often adhering shells attached to them, and have been bored by perforating
mollusca. The solid surface of the inferior limestone has also been bored, so as
to exhibit cylindrical and pear-shaped cavities, as at c, the work of saxicavous
mollusca; and many rents, as at b, which descend several feet or yards into the
limestone, have been filled with sand and shells, similar to those in the
Strata are said to overlap when an upper bed extends beyond the limits of a
lower one. This may be produced in various ways; as, for example, when
alterations of physical geography cause the arms of a river or channels of
discharge to vary, so that sediment brought down is deposited over a wider area
than before, or when the sea-bottom has been raised up and again depressed
without disturbing the horizontal position of the strata. In this case the newer
strata may rest for the most part conformably on the older, but, extending
farther, pass over their edges. Every intermediate state between unconformable
and over-lapping beds may occur, because there may be every gradation between a
slight derangement of position, and a considerable disturbance and denudation of
the older formation before the newer beds come on.
Its Amount more than equal to the entire Mass of Stratified Deposits in the
Action of the Wind.
Action of Running Water.
Different Ages of Alluvium.
Denuding Power of Rivers affected by Rise or Fall of Land.
Denuding Power of the Ocean during Emergence of Land.
Denudation, which has been occasionally spoken of in the preceding chapters, is
the removal of solid matter by water in motion, whether of rivers or of the
waves and currents of the sea, and the consequent laying bare of some inferior
rock. This operation has exerted an influence on the structure of the earth's
crust as universal and important as sedimentary deposition itself; for
denudation is the necessary antecedent of the production of all new strata of
mechanical origin. The formation of every new deposit by the transport of
sediment and pebbles necessarily implies that there has been, somewhere else, a
grinding down of rock into rounded fragments, sand, or mud, equal in quantity to
the new strata. All deposition, therefore, except in the case of a shower of
volcanic ashes, and the outflow of lava, and the growth of certain organic
formations, is the sign of superficial waste going on contemporaneously, and to
an equal amount, elsewhere. The gain at one point is no more than sufficient to
balance the loss at some other. Here a lake has grown shallower, there a ravine
has been deepened. Here the depth of the sea has been augmented by the removal
of a sandbank during a storm, there its bottom has been raised and shallowed by
the accumulation in its bed of the same sand transported from the bank.
When we see a stone building, we know that somewhere, far or near, a quarry has
been opened. The courses of stone in the building may be compared to successive
strata, the quarry to a ravine or valley which has suffered denudation. As the
strata, like the courses of hewn stone, have been laid one upon another
gradually, so the excavation both of the valley and quarry have been gradual. To
pursue the comparison still farther, the superficial heaps of mud, sand, and
gravel, usually called alluvium, may be likened to the rubbish of a quarry which
has been rejected as useless by the workmen, or has fallen upon the road between
the quarry and the building, so as to lie scattered at random over the ground.
But we occasionally find in a conglomerate large rounded pebbles of an older
conglomerate, which had previously been derived from a variety of different
rocks. In such cases we are reminded that, the same materials having been used
over and over again, it is not enough to affirm that the entire mass of
stratified deposits in the earth's crust affords a monument and measure of the
denudation which has taken place, for in truth the quantity of matter now extant
in the form of stratified rock represents but a fraction of the material removed
by water and redeposited in past ages.
Denudation may be divided into subaerial, or the action of wind, rain, and
rivers; and submarine, or that effected by the waves of the sea, and its tides
and currents. With the operation of the first of these we are best acquainted,
and it may be well to give it our first attention.
ACTION OF THE WIND.
In desert regions where no rain falls, or where, as in parts of the Sahara, the
soil is so salt as to be without any covering of vegetation, clouds of dust and
sand attest the power of the wind to cause the shifting of the unconsolidated or
In examining volcanic countries I have been much struck with the great
superficial changes brought about by this power in the course of centuries. The
highest peak of Madeira is about 6050 feet above the sea, and consists of the
skeleton of a volcanic cone now 250 feet high, the beds of which once dipped
from a centre in all directions at an angle of more than 30 degrees. The summit
is formed of a dike of basalt with much olivine, fifteen feet wide, apparently
the remains of a column of lava which once rose to the crater. Nearly all the
scoriae of the upper part of the cone have been swept away, those portions only
remaining which were hardened by the contact or proximity of the dike. While I
was myself on this peak on January 25, 1854, I saw the wind, though it was not
stormy weather, removing sand and dust derived from the decomposing scoriae.
There had been frost in the night, and some ice was still seen in the crevices
of the rock.
On the highest platform of the Grand Canary, at an elevation of 6000 feet, there
is a cylindrical column of hard lava, from which the softer matter has been
carried away; and other similar remnants of the dikes of cones of eruption
attest the denuding power of the wind at points where running water could never
have exerted any influence. The waste effected by wind aided by frost and snow,
may not be trifling, even in a single winter, and when multiplied by centuries
may become indefinitely great.
ACTION OF RUNNING WATER.
(FIGURE 80. Section through several eroded formations.
a. Older alluvium or drift.
b. Modern alluvium.)
There are different classes of phenomena which attest in a most striking manner
the vast spaces left vacant by the erosive power of water. I may allude, first,
to those valleys on both sides of which the same strata are seen following each
other in the same order, and having the same mineral composition and fossil
contents. We may observe, for example, several formations, as Nos. 1, 2, 3, 4,
in the diagram (Figure 80): No. 1, conglomerate, No. 2, clay, No. 3, grit, and
No. 4, limestone, each repeated in a series of hills separated by valleys
varying in depth. When we examine the subordinate parts of these four
formations, we find, in like manner, distinct beds in each, corresponding, on
the opposite sides of the valleys, both in composition and order of position. No
one can doubt that the strata were originally continuous, and that some cause
has swept away the portions which once connected the whole series. A torrent on
the side of a mountain produces similar interruptions; and when we make
artificial cuts in lowering roads, we expose, in like manner, corresponding beds
on either side. But in nature, these appearances occur in mountains several
thousand feet high, and separated by intervals of many miles or leagues in
In the "Memoirs of the Geological Survey of Great Britain" (volume 1), Professor
Ramsay has shown that the missing beds, removed from the summit of the Mendips,
must have been nearly a mile in thickness; and he has pointed out considerable
areas in South Wales and some of the adjacent counties of England, where a
series of primary (or palaeozoic) strata, no less than 11,000 feet in thickness,
have been stripped off. All these materials have of course been transported to
new regions, and have entered into the composition of more modern formations. On
the other hand, it is shown by observations in the same "Survey," that the
Palaeozoic strata are from 20,000 to 30,000 feet thick. It is clear that such
rocks, formed of mud and sand, now for the most part consolidated, are the
monuments of denuding operations, which took place on a grand scale at a very
remote period in the earth's history. For, whatever has been given to one area
must always have been borrowed from another; a truth which, obvious as it may
seem when thus stated, must be repeatedly impressed on the student's mind,
because in many geological speculations it is taken for granted that the
external crust of the earth has been always growing thicker in consequence of
the accumulation, period after period, of sedimentary matter, as if the new
strata were not always produced at the expense of pre-existing rocks, stratified
or unstratified. By duly reflecting on the fact that all deposits of mechanical
origin imply the transportation from some other region, whether contiguous or
remote, of an equal amount of solid matter, we perceive that the stony exterior
of the planet must always have grown thinner in one place, whenever, by
accessions of new strata, it was acquiring thickness in another.
It is well known that generally at the mouths of large rivers, deltas are
forming and the land is encroaching upon the sea; these deltas are monuments of
recent denudation and deposition; and it is obvious that if the mud, sand, and
gravel were taken from them and restored to the continents they would fill up a
large part of the gullies and valleys which are due to the excavating and
transporting power of torrents and rivers.
Between the superficial covering of vegetable mould and the subjacent rock there
usually intervenes in every district a deposit of loose gravel, sand, and mud,
to which when it occurs in valleys the name of alluvium has been popularly
applied. The term is derived from alluvio, an inundation, or alluo, to wash,
because the pebbles and sand commonly resemble those of a river's bed or the mud
and gravel washed over low lands by a flood.
In the course of those changes in physical geography which may take place during
the gradual emergence of the bottom of the sea and its conversion into dry land,
any spot may either have been a sunken reef, or a bay, or estuary, or sea-shore,
or the bed of a river. The drainage, moreover, may have been deranged again and
again by earthquakes, during which temporary lakes are caused by landslips, and
partial deluges occasioned by the bursting of the barriers of such lakes. For
this reason it would be unreasonable to hope that we should ever be able to
account for all the alluvial phenomena of each particular country, seeing that
the causes of their origin are so various. Besides, the last operations of water
have a tendency to disturb and confound together all pre-existing alluviums.
Hence we are always in danger of regarding as the work of a single era, and the
effect of one cause, what has in reality been the result of a variety of
distinct agents, during a long succession of geological epochs. Much useful
instruction may therefore be gained from the exploration of a country like
Auvergne, where the superficial gravel of very different eras happens to have
been preserved and kept separate by sheets of lava, which were poured out one
after the other at periods when the denudation, and probably the upheaval, of
rocks were in progress. That region had already acquired in some degree its
present configuration before any volcanoes were in activity, and before any
igneous matter was superimposed upon the granitic and fossiliferous formations.
The pebbles therefore in the older gravels are exclusively constituted of
granite and other aboriginal rocks; and afterwards, when volcanic vents burst
forth into eruption, those earlier alluviums were covered by streams of lava,
which protected them from intermixture with gravel of subsequent date. In the
course of ages, a new system of valleys was excavated, so that the rivers ran at
lower levels than those at which the first alluviums and sheets of lava were
formed. When, therefore, fresh eruptions gave rise to new lava, the melted
matter was poured out over lower grounds; and the gravel of these plains
differed from the first or upland alluvium, by containing in it rounded
fragments of various volcanic rocks, and often fossil bones belonging to species
of land animals different from those which had previously flourished in the same
country and been buried in older gravels.
(FIGURE 81. Lavas of Auvergne resting on alluviums of different ages.)
Figure 81 will explain the different heights at which beds of lava and gravel,
each distinct from the other in composition and age, are observed, some on the
flat tops of hills, 700 or 800 feet high, others on the slope of the same hills,
and the newest of all in the channel of the existing river where there is
usually gravel alone, although in some cases a narrow strip of solid lava shares
the bottom of the valley with the river.
The proportion of extinct species of quadrupeds is more numerous in the fossil
remains of the gravel No. 1 than in that indicated as No. 2; and in No. 3 they
agree more closely, sometimes entirely, with those of the existing fauna. The
usual absence or rarity of organic remains in beds of loose gravel and sand is
partly owing to the friction which originally ground down the rocks into small
fragments, and partly to the porous nature of alluvium, which allows the free
percolation through it of rain-water, and promotes the decomposition and removal
of fossil remains.
The loose transported matter on the surface of a large part of the land now
existing in the temperate and arctic regions of the northern hemisphere, must be
regarded as being in a somewhat exceptional state, in consequence of the
important part which ice has played in comparatively modern geological times.
This subject will be more specially alluded to when we describe, in the eleventh
chapter, the deposits called "glacial."
DENUDING POWER OF RIVERS AFFECTED BY RISE OR FALL OF LAND.
It has long been a matter of common observation that most rivers are now cutting
their channels through alluvial deposits of greater depth and extent than could
ever have been formed by the present streams. From this fact it has been
inferred that rivers in general have grown smaller, or become less liable to be
flooded than formerly. It may be true that in the history of almost every
country the rivers have been both larger and smaller than they are at the
present moment. For the rainfall in particular regions varies according to
climate and physical geography, and is especially governed by the elevation of
the land above the sea, or its distance from it and other conditions equally
fluctuating in the course of time. But the phenomenon alluded to may sometimes
be accounted for by oscillations in the level of the land, experienced since the
existing valleys originated, even where no marked diminution in the quantity of
rain and in the size of the rivers has occurred.
We know that many large areas of land are rising and others sinking, and unless
it could be assumed that both the upward and downward movements are everywhere
uniform, many of the existing hydrographical basins ought to have the appearance
of having been temporary lakes first filled with fluviatile strata and then
Suppose, for example, part of a continent, comprising within it a large
hydrographical basin like that of the Mississippi, to subside several inches or
feet in a century, as the west coast of Greenland, extending 600 miles north and
south, has been sinking for three or four centuries, between the latitudes 60
and 69 degrees N. (Principles of Geology 7th edition page 506; 10th edition
volume 2 page 196.) It will rarely happen that the rate of subsidence will be
everywhere equal, and in many cases the amount of depression in the interior
will regularly exceed that of the region nearer the sea. Whenever this happens,
the fall of the waters flowing from the upland country will be diminished, and
each tributary stream will have less power to carry its sand and sediment into
the main river, and the main river less power to convey its annual burden of
transported matter to the sea. All the rivers, therefore, will proceed to fill
up partially their ancient channels, and, during frequent inundations, will
raise their alluvial plains by new deposits. If then the same area of land be
again upheaved to its former height, the fall, and consequently the velocity, of
every river will begin to augment. Each of them will be less given to overflow
its alluvial plain; and their power of carrying earthy matter seaward, and of
scouring out and deepening their channels, will be sustained till, after a lapse
of many thousand years, each of them has eroded a new channel or valley through
a fluviatile formation of comparatively modern date. The surface of what was
once the river-plain at the period of greatest depression, will then remain
fringing the valley-sides in the form of a terrace apparently flat, but in
reality sloping down with the general inclination of the river. Everywhere this
terrace will present cliffs of gravel and sand, facing the river. That such a
series of movements has actually taken place in the main valley of the
Mississippi and in its tributary valleys during oscillations of level, I have
endeavoured to show in my description of that country (Second Visit to the
United States volume 1 chapter 34.); and the fresh-water shells of existing
species and bones of land quadrupeds, partly of extinct races, preserved in the
terraces of fluviatile origin, attest the exclusion of the sea during the whole
process of filling up and partial re-excavation.
Part of the action of the waves between high and low watermark must be included
in subaerial denudation, more especially as the undermining of cliffs by the
waves is facilitated by land-springs, and these often lead to the sliding down
of great masses of land into the sea. Along our coasts we find numerous
submerged forests, only visible at low water, having the trunks of the trees
erect and their roots attached to them and still spreading through the ancient
soil as when they were living. They occur in too many places, and sometimes at
too great a depth, to be explained by a mere change in the level of the tides,
although as the coasts waste away and alter in shape, the height to which the
tides rise and fall is always varying, and the level of high tide at any given
point may, in the course of many ages, differ by several feet or even fathoms.
It is this fluctuation in the height of the tides, and the erosion and
destruction of the sea-coast by the waves, that makes it exceedingly difficult
for us in a few centuries, or even perhaps in a few thousand years, to determine
whether there is a change by subterranean movement in the relative level of sea
We often behold, as on the coasts of Devonshire and Pembrokeshire, facts which
appear to lead to opposite conclusions. In one place a raised beach with marine
littoral shells, and in another immediately adjoining a submerged forest. These
phenomena indicate oscillations of level, and as the movements are very gradual,
they must give repeated opportunities to the breakers to denude the land which
is thus again and again exposed to their fury, although it is evident that the
submergence is sometimes effected in such a manner as to allow the trees which
border the coast not to be carried away.
In countries where hard limestone rocks abound, inland cliffs have often
retained faithfully for ages the characters which they acquired when they
constituted the boundary of land and sea. Thus, in the Morea, no less than three
or even four ranges of cliffs are well-preserved, rising one above the other at
different distances from the actual shore, the summit of the highest and oldest
occasionally attaining 1000 feet in elevation. A consolidated beach with marine
shells is usually found at the base of each cliff, and a line of littoral
caverns. These ranges of cliff probably imply pauses in the process of upheaval
when the waves and currents had time to undermine and clear away considerable
masses of rock.
But the beginner should be warned not to expect to find evidence of the former
sojourn of the sea on all those lands which we are nevertheless sure have been
submerged at periods comparatively modern; for notwithstanding the enduring
nature of the marks left by littoral action on some rocks, especially
limestones, we can by no means detect sea-beaches and inland cliffs everywhere.
On the contrary, they are, upon the whole, extremely partial, and are often
entirely wanting in districts composed of argillaceous and sandy formations,
which must, nevertheless, have been upheaved at the same time, and by the same
intermittent movements, as the adjoining harder rocks.
Besides the inland cliffs above alluded to which mark the ancient limits of the
sea, there are other abrupt terminations of rocks of various kinds which
resemble sea-cliffs, but which have in reality been due to subaerial denudation.
These have been called "escarpments," a term which it is useful to confine to
the outcrop of particular formations having a scarped outline, as distinct from
cliffs due to marine action.
I formerly supposed that the steep line of cliff-like slopes seen along the
outcrop of the chalk, when we follow the edge of the North or South Downs, was
due to marine action; but Professor Ramsay has shown (Physical Geography and
Geology of Great Britain page 78 1864.) that the present outline of the physical
geography is more in favour of the idea of the escarpments having been due to
gradual waste since the rocks were exposed in the atmosphere to the action of
rain and rivers.
Mr. Whittaker has given a good summary of the grounds for ascribing these
apparent sea-cliffs to waste in the open air. 1. There is an absence of all
signs of ancient sea-beaches or littoral deposits at the base of the escarpment.
2. Great inequality is observed in the level of the base line. 3. The
escarpments do not intersect, like sea-cliffs, a series of distinct rocks, but
are always confined to the boundary-line of the same formation. 4. There are
sometimes different contiguous and parallel escarpments-- those, for example, of
the greensand and chalk-- which are so near each other, and occasionally so
similar in altitude, that we can not imagine any existing archipelago if
converted into dry land to present a like outline.
The above theory is by no means inconsistent with the opinion that the limits of
the outcrop of the chalk and greensand which the escarpments now follow, were
originally determined by marine denudation. When the south-east of England last
emerged from beneath the level of the sea, it was acted upon, no doubt, by the
tide, waves, and currents, and the chalk would form from the first a mass
projecting above the more destructible clay called Gault. Still the present
escarpments so much resembling sea-cliffs have no doubt, for reasons above
stated, derived their most characteristic features subsequently to emergence
from subaerial waste by rain and rivers.
When we attempt to estimate the amount of submarine denudation, we become
sensible of the disadvantage under which we labour from our habitual incapacity
of observing the action of marine currents on the bed of the sea. We know that
the agitation of the waves, even during storms, diminishes at a rapid rate, so
as to become very insignificant at the depth of a few fathoms, and is quite
imperceptible at the depth of about sixteen fathoms; but when large bodies of
water are transferred by a current from one part of the ocean to another, they
are known to maintain at great depths such a velocity as must enable them to
remove the finer, and sometimes even the coarser, materials of the rocks over
which they flow. As the Mississippi when more than 150 feet deep can keep open
its channel and even carry down gravel and sand to its delta, the surface
velocity being not more than two or three miles an hour, so a gigantic current,
like the Gulf Stream, equal in volume to many hundred Mississippis, and having
in parts a surface velocity of more than three miles, may act as a propelling
and abrading power at still greater depths. But the efficacy of the sea as a
denuding agent, geologically considered, is not dependent on the power of
currents to preserve at great depths a velocity sufficient to remove sand and
mud, because, even where the deposition or removal of sediment is not in
progress, the depth of water does not remain constant throughout geological
time. Every page of the geological record proves to us that the relative levels
of land and sea, and the position of the ocean and of continents and islands,
has been always varying, and we may feel sure that some portions of the
submarine area are now rising and others sinking. The force of tidal and other
currents and of the waves during storms is sufficient to prevent the emergence
of many lands, even though they may be undergoing continual upheaval. It is not
an uncommon error to imagine that the waste of sea-cliffs affords the measure of
the amount of marine denudation of which it probably constitutes an
That great shoal called the Dogger-bank, about sixty miles east of the coast of
Northumberland, and occupying an area about as large as Wales, has nowhere a
depth of more than ninety feet, and in its shallower parts is less than forty
feet under water. It might contribute towards the safety of the navigation of
our seas to form an artificial island, and to erect a light-house on this bank;
but no engineer would be rash enough to attempt it, as he would feel sure that
the ocean in the first heavy gale would sweep it away as readily as it does
every temporary shoal that accumulates from time to time around a sunk vessel on
the same bank. (Principles 10th edition volume 1 page 569.)
No observed geographical changes in historical times entitle us to assume that
where upheaval may be in progress it proceeds at a rapid rate. Three or four
feet rather than as many yards in a century may probably be as much as we can
reckon upon in our speculations; and if such be the case, the continuance of the
upward movement might easily be counteracted by the denuding force of such
currents aided by such waves as, during a gale, are known to prevail in the
German Ocean. What parts of the bed of the ocean are stationary at present, and
what areas may be rising or sinking, is a matter of which we are very ignorant,
as the taking of accurate soundings is but of recent date.
The great bank of Newfoundland may be compared in size to the whole of England.
This part of the bottom of the Atlantic is surrounded on three sides by a
rapidly deepening ocean, the bank itself being from twenty to fifty fathoms (or
from 120 to 300 feet) under water. We are unable to determine by the comparison
of different charts made at distant periods, whether it is undergoing any change
of level, but if it be gradually rising we can not anticipate on that account
that it will become land, because the breakers in an open sea would exercise a
prodigious force even on solid rock brought up to within a few yards of the
surface. We know, for example, that when a new volcanic island rose in the
Mediterranean in 1831, the waves were capable in a few years of reducing it to a
In the same way currents which flow over the Newfoundland bank a great part of
the year at the rate of two miles an hour, and are known to retain a
considerable velocity to near the bottom, may carry away all loose sand and mud,
and make the emergence of the shoal impossible, in spite of the accessions of
mud, sand, and boulders derived occasionally from melting icebergs which, coming
from the northern glaciers, are frequently stranded on various parts of the
bank. They must often leave at the bottom large erratic blocks which the marine
currents may be incapable of moving, but the same rocky fragments may be made to
sink by the undermining of beds consisting of finer matter on which the blocks
and gravel repose. In this way gravel and boulders may continue to overspread a
submarine bottom after the latter has been lowered for hundreds of feet, the
surface never having been able to emerge and become land. It is by no means
improbable that the annual removal of an average thickness of half an inch of
rock might counteract the ordinary upheaval which large submarine areas are
undergoing; and the real enigma which the geologist has to solve is not the
extensive denudation of the white chalk or of our tertiary sands and clays, but
the fact that such incoherent materials have ever succeeded in lifting up their
heads above water in an open sea. Why were they not swept away during storms
into some adjoining abysses, the highest parts of each shoal being always planed
off down to the depth of a few fathoms? The hardness and toughness of some rocks
already exposed to windward and acting as breakwaters may perhaps have assisted;
nor must we forget the protection afforded by a dense and unbroken covering of
barnacles, limpets, and other creatures which flourish most between high and low
water and shelter some newly risen coasts from the waves.
JOINT ACTION OF DENUDATION, UPHEAVAL, AND SUBSIDENCE IN REMODELLING THE EARTH'S
How we obtain an Insight at the Surface, of the Arrangement of Rocks at great
Why the Height of the successive Strata in a given Region is so disproportionate
to their Thickness.
Computation of the average annual Amount of subaerial Denudation.
Antagonism of Volcanic Force to the Levelling Power of running Water.
How far the Transfer of Sediment from the Land to a neighbouring Sea-bottom may
affect Subterranean Movements.
Permanence of Continental and Oceanic Areas.
HOW WE OBTAIN AN INSIGHT AT THE SURFACE, OF THE ARRANGEMENT OF ROCKS AT GREAT
The reader has been already informed that, in the structure of the earth's
crust, we often find proofs of the direct superposition of marine to fresh-water
strata, and also evidence of the alternation of deep-sea and shallow-water
formations. In order to explain how such a series of rocks could be made to form
our present continents and islands, we have not only to assume that there have
been alternate upward and downward movements of great vertical extent, but that
the upheaval in the areas which we at present inhabit has, in later geological
times, sufficiently predominated over subsidence to cause these portions of the
earth's crust to be land instead of sea. The sinking down of a delta beneath the
sea-level may cause strata of fluviatile or even terrestrial origin, such as
peat with trees proper to marshes, to be covered by deposits of deep-sea origin.
There is also no end to the thickness of mud and sand which may accumulate in
shallow water, provided that fresh sediment is brought down from the wasting
land at a rate corresponding to that of the sinking of the bed of the sea. The
latter, again, may sometimes sink so fast that the earthy matter, being
intercepted in some new landward depression, may never reach its former resting-
place, where, the water becoming clear may favour the growth of shells and
corals, and calcareous rocks of organic origin may thus be superimposed on
The succession of strata here alluded to would be consistent with the occurrence
of gradual downward and upward movements of the land and bed of the sea without
any disturbance of the horizontality of the several formations. But the
arrangement of rocks composing the earth's crust differs materially from that
which would result from a mere series of vertical movements. Had the volcanic
forces been confined to such movements, and had the stratified rocks been first
formed beneath the sea and then raised above it, without any lateral
compression, the geologist would never have obtained an insight into the
monuments of various ages, some of extremely remote antiquity.
What we have said in Chapter 5 of dip and strike, of the folding and inversion
of strata, of anticlinal and synclinal flexures, and in Chapter 6 of denudation
at different periods, whether subaerial or submarine, must be understood before
the student can comprehend what may at first seem to him an anomaly, but which
it is his business particularly to understand. I allude to the small height
above the level of the sea attained by strata often many miles in thickness, and
about the chronological succession of which, in one and the same region, there
is no doubt whatever. Had stratified rocks in general remained horizontal, the
waves of the sea would have been enabled during oscillations of level to plane
off entirely the uppermost beds as they rose or sank during the emergence or
submergence of the land. But the occurrence of a series of formations of widely
different ages, all remaining horizontal and in conformable stratification, is
exceptional, and for this reason the total annihilation of the uppermost strata
has rarely taken place. We owe, indeed, to the side way movements of LATERAL
COMPRESSION those anticlinal and synclinal curves of the beds already described
(Figure 55 Chapter 4), which, together with denudation, subaerial and submarine,
enable us to investigate the structure of the earth's crust many miles below
those points which the miner can reach. I have already shown in Figure 56
Chapter 4, how, at St. Abb's Head, a series of strata of indefinite thickness
may become vertical, and then denuded, so that the edges of the beds alone shall
be exposed to view, the altitude of the upheaved ridges being reduced to a
moderate height above the sea-level; and it may be observed that although the
incumbent strata of Old Red Sandstone are in that place nearly horizontal, yet
these same newer beds will in other places be found so folded as to present
vertical strata, the edges of which are abruptly cut off, as in 2, 3, 4 on the
right-hand side of the diagram, Figure 55 Chapter 4.
WHY THE HEIGHT OF THE SUCCESSIVE STRATA IN A GIVEN REGION IS SO DISPROPORTIONATE
TO THEIR THICKNESS.
We can not too distinctly bear in mind how dependent we are on the joint action
of the volcanic and aqueous forces, the one in disturbing the original position
of rocks, and the other in destroying large portions of them, for our power of
consulting the different pages and volumes of those stony records of which the
crust of the globe is composed. Why, it may be asked, if the ancient bed of the
sea has been in many regions uplifted to the height of two or three miles, and
sometimes twice that altitude, and if it can be proved that some single
formations are of themselves two or three miles thick, do we so often find
several important groups resting one upon the other, yet attaining only the
height of a few hundred feet above the level of the sea?
The American geologists, after carefully studying the Allegheny or Appalachian
mountains, have ascertained that the older fossiliferous rocks of that chain
(from the Silurian to the Carboniferous inclusive) are not less than 42,000 feet
thick, and if they were now superimposed on each other in the order in which
they were thrown down, they ought to equal in height the Himalayas with the Alps
piled upon them. Yet they rarely reach an altitude of 5000 feet, and their
loftiest peaks are no more than 7000 feet high. The Carboniferous strata forming
the highest member of the series, and containing beds of coal, can be shown to
be of shallow-water origin, or even sometimes to have originated in swamps in
the open air. But what is more surprising, the lowest part of this great
Palaeozoic series, instead of having been thrown down at the bottom of an abyss
more than 40,000 feet deep, consists of sediment (the Potsdam sandstone),
evidently spread out on the bottom of a shallow sea, on which ripple-marked
sands were occasionally formed. This vast thickness of 40,000 feet is not
obtained by adding together the maximum density attained by each formation in
distant parts of the chain, but by measuring the successive groups as they are
exposed in a very limited area, and where the denuded edges of the vertical
strata forming the parallel folds alluded to in Chapter 5 "crop out" at the
surface. Our attention has been called by Mr. James Hall, Palaeontologist of New
York, to the fact that these Palaeozoic rocks of the Appalachian chain, which
are of such enormous density, where they are almost entirely of mechanical
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