The Student's Elements of Geology
by
Sir Charles Lyell
Part 9 out of 14
As to the climate of the Coal, the Ferns and the Coniferae are perhaps the two
classes of plants which may be most relied upon as leading us to safe
conclusions, as the genera are nearly allied to living types. All botanists
admit that the abundance of ferns implies a moist atmosphere. But the coniferae,
says Hooker, are of more doubtful import, as they are found in hot and dry, and
in cold and dry climates; in hot and moist, and in cold and moist regions. In
New Zealand the coniferae attain their maximum in numbers, constituting 1/62
part of all the flowering plants; whereas in a wide district around the Cape of
Good Hope they do not form 1/1600 of the phenogamic flora. Besides the conifers,
many species of ferns flourish in New Zealand, some of them arborescent,
together with many lycopodiums; so that a forest in that country may make a
nearer approach to the carboniferous vegetation than any other now existing on
the globe.
MARINE FAUNA OF THE CARBONIFEROUS PERIOD.
It has already been stated that the Carboniferous or Mountain Limestone
underlies the coal-measures in the South of England and Wales, whereas in the
North, and in Scotland, marine calcareous rocks partly of the age of the
Mountain Limestone alternate with shales and sandstones, containing seams of
coal. In its most calcareous form the Mountain Limestone is destitute of land-
plants, and is loaded with marine remains-- the greater part, indeed, of the
rock being made up bodily of crinoids, corals, and bryozoa with interspersed
mollusca.
CORALS.
(FIGURE 474. Palaeozoic type of lamelliferous cup-shaped Coral. Order ZOANTHARIA
RUGOSA, Milne Edwards and Jules Haime.
a. Vertical section of Campophyllum flexuosum, (Cyathophyllum, Goldfuss); 1/2
natural size: from the Devonian of the Eifel. The lamellae are seen around the
inside of the cup; the walls consist of cellular tissue; and large transverse
plates, called tubulae, divide the interior into chambers.
b. Arrangement of the lamellae in Polycoelia profunda, Germar, sp.; natural
size: from the Magnesian Limestone, Durham.
This diagram shows the quadripartite arrangement of the primary septa,
characteristic of palaeozoic corals, there being four principal and eight
intermediate lamellae, the whole number in this type being always a multiple of
four.
c. Stauria astraeiformis, Milne Edwards. Young group, natural size. Upper
Silurian, Gothland. The lamellae or septal system in each cup are divided by
four prominent ridges into four groups.)
(FIGURE 475. Neozoic type of lamelliferous cup-shaped Coral. Order ZOANTHARIA
APOROSA, M. Edwards and J. Haime.
a. Parasmilia centralis, Mantell, sp. Vertical section; natural size. Upper
Chalk, Gravesend. In this type the lamellae are massive, and extend to the axis
or columella composed of loose cellular tissue, without any transverse plates
like those in Figure 474, a.
b. Cyathina Bowerbankii, Ed. and H. Transverse section, enlarged. Gault,
Folkestone. In this coral the primary septa are a multiple of six. The twelve
principal plates reach the columella, and between each pair there are three
secondaries, in all forty-eight. The short intermediate plates which proceed
from the columella are not counted. They are called pali.
c. Fungia patellaris, Lamarck. Recent; very young state. Diagram of its six
primary and six secondary septa, magnified. The sextuple arrangement is always
more manifest in the young than in the adult state.)
The corals deserve especial notice, as the cup-and-star corals, which have the
most massive and stony skeletons, display peculiarities of structure by which
they may be distinguished generally, as MM. Milne Edwards and Haime first
pointed out, from all species found in strata newer than the Permian. There is,
in short, an ancient or PALAEOZOIC, and a modern or NEOZOIC type, if, by the
latter term, we designate (as proposed by Professor E. Forbes) all strata from
the triassic to the most modern, inclusive. The accompanying diagrams (Figures
474, 475) may illustrate these types.
It will be seen that the more ancient corals have what is called a quadripartite
arrangement of the chief plates or LAMELLAE-- parts of the skeleton which
support the organs of reproduction. The number of these lamellae in the
Palaeozoic type is 4, 8, 16, etc.; while in the Neozoic type the number is 6,
12, 24, or some other multiple of six; and this holds good, whether they be
simple forms, as in Figures 474, a, and 475, a, or aggregate clusters of
corallites, as in 474, c. But further investigations have shown in this, as in
all similar grand generalisations in natural history, that there are exceptions
to the rule. Thus in the Lower Greensand Holocystis elegans (Ed. and H.) and
other forms have the Palaeozoic type, and Dr. Duncan has shown to what extent
the Neozoic forms penetrate downward into the Carboniferous and Devonian rocks.
(FIGURE 476. Lithostrotion basaltiforme, Phil. sp. (Lithostrotion striatum,
Fleming; Astraea basaltiformis, Conyb. and Phill.). England, Ireland, Russia,
Iowa, and westward of the Mississippi, United States. (D.D. Owen.)
(FIGURE 477. Lonsdaleia floriformis, Martin, sp., M. Edwards. (Lithostrotion
floriforme, Fleming. Strombodes.)
a. Young specimen, with buds or corallites on the disk, illustrating calicular
gemmation.
b. Part of a full-grown compound mass. Bristol, etc.; Russia.)
From a great number of lamelliferous corals met with in the Mountain Limestone,
two species (Figures 476, 477) have been selected, as having a very wide range,
extending from the eastern borders of Russia to the British Isles, and being
found almost everywhere in each country. These fossils, together with numerous
species of Zaphrentis, Amplexus, Cyathophyllum, Clisiophyllum, Syringopora, and
Michelinia, form a group of rugose corals widely different from any that
followed them. (For figures of these corals, see Palaeontographical Society's
Monographs 1852.)
BRYOZOA AND CRINOIDEA.
(FIGURE 478. Cyathocrinus planus, Miller. Body and arms. Mountain Limestone.)
(FIGURE 479. Cyathocrinus caryocrinoides, M'Coy.
a. Surface of one of the joints of the stem.
b. Pelvis or body; called also calyx or cup.
c. One of the pelvic plates.)
Of the Bryozoa, the prevailing forms are Fenestella, Hemitrypa, and Polypora,
and these often form considerable beds. Their net-like fronds are easily
recognised. Crinoidea are also numerous in the Mountain Limestone (see Figures
478, 479), two genera, Pentremites and Codonaster, being peculiar to this
formation in Europe and North America.
(FIGURE 480. Palaechinus gigas, M'Coy. Reduced one-third. Mountain Limestone.
Ireland.)
In the greater part of them, the cup or pelvis, Figure 479, b, is greatly
developed in size in proportion to the arms, although this is not the case in
Figure 478. The genera Poteriocrinus, Cyathocrinus, Pentremites, Actinocrinus,
and Platycrinus, are all of them characteristic of this formation. Other
Echinoderms are rare, a few Sea-Urchins only being known: these have a complex
structure, with many more plates on their surface than are seen in the modern
genera of the same group. One genus, the Palaechinus (Figure 480), is the
analogue of the modern Echinus, but has four, five, or six rows of plates in the
interambulacral region or area, whereas the modern genera have only two. The
other, Archaeocidaris, represents, in like manner, the Cidaris of the present
seas.
MOLLUSCA.
(FIGURE 481. Productus semireticulatus, Martin, sp. (P. antiquatus, Sowerby.)
Mountain Limestone. England, Russia, the Andes, etc.)
(FIGURE 482. Spirifera trigonalis, Martin, sp. Mountain Limestone. Derbyshire,
etc.)
(FIGURE 483. Spirifera glabra, Martin, sp. Mountain Limestone.)
The British Carboniferous mollusca enumerated by Mr. Etheridge comprise 653
species referable to 86 genera, occurring chiefly in the Mountain Limestone.
(Quarterly Geological Journal volume 23 page 674 1867.) Of this large number
only 40 species are common to the underlying Devonian rocks, 9 of them being
Cephalopods, 7 Gasteropods, and the rest bivalves, chiefly Brachiopoda (or
Palliobranchiates). This latter group constitutes the larger part of the
Carboniferous Mollusca, 157 species being known in Great Britain alone, and it
will be found to increase in importance in the fauna of the primary rocks the
lower we descend in the series. Perhaps the most characteristic shells of the
formation are large species of Productus, such as P. giganteus, p.
hemisphericus, P. semireticulatus (Figure 481), and P. scabriculus. Large
plaited spirifers, as Spirifera striata, S. rotundata, and S. trigonalis (Figure
482), also abound; and smooth species, such as Spirifera glabra (Figure 483),
with its numerous varieties.
(FIGURE 484. Terebratula hastata, Sowerby, with radiating bands of colour.
Mountain Limestone. Derbyshire, Ireland, Russia, etc.)
(FIGURE 485. Aviculopecten sublobatus, Phill. Mountain Limestone. Derbyshire,
Yorkshire.)
(FIGURE 486. Pleurotomaria carinata, Sowerby. (P. flammigera, Phillips).
Mountain Limestone. Derbyshire, etc.)
Among the brachiopoda, Terebratula hastata (Figure 484) deserves mention, not
only for its wide range, but because it often retains the pattern of the
original coloured stripes which ornamented the living shell. These coloured
bands are also preserved in several lamellibranchiate bivalves, as in
Aviculopecten (Figure 485), in which dark stripes alternate with a light ground.
In some also of the spiral univalves the pattern of the original painting is
distinctly retained, as in Pleurotomaria (Figure 486), which displays wavy
blotches, resembling the colouring in many recent trochidae.
(FIGURE 487. Euomphalus pentangulatus, Sowerby. Mountain Limestone.
a. Upper side.
b. Lower or umbilical side.
c. View showing mouth, which is less pentagonal in older individuals.
d. View of polished section, showing internal chambers.)
Some few of the carboniferous mollusca, such as Avicula, Nucula (sub-genus
Ctenodonta), Solemya, and Lithodomus, belong no doubt to existing genera; but
the majority, though often referred to as living types, such as Isocardia,
Turritella, and Buccinum, belong really to forms which appear to have become
extinct at the close of the Palaeozoic epoch. Euomphalus is a characteristic
univalve shell of this period. In the interior it is divided into chambers
(Figure 487, d), the septa or partitions not being perforated as in
foraminiferous shells, or in those having siphuncles, like the Nautilus. The
animal appears to have retreated at different periods of its growth from the
internal cavity previously formed, and to have closed all communication with it
by a septum. The number of chambers is irregular, and they are generally wanting
in the innermost whorl. The animal of the recent Turritella communis partitions
off in like manner as it advances in age a part of its spire, forming a shelly
septum.
(FIGURE 488. Bellerophon costatus, Sowerby. Mountain Limestone.)
More than twenty species of the genus Bellerophon (see Figure 488), a shell like
the living Argonaut without chambers, occur in the Mountain Limestone. The genus
is not met with in strata of later date. It is most generally regarded as
belonging to the pelagic Nucleobranchiata and the family Atlantidae, partly
allied to the Glass-Shell, Carinaria; but by some few it is thought to be a
simple form of Cephalopod.
(FIGURE 489. Portion of Orthoceras laterale. Phill. Mountain Limestone.)
(FIGURE 490. Goniatites crenistra, Phillips. Mountain Limestone. North America,
Britain, Germany, etc.
a. Lateral view.
b. Front view, showing the mouth.)
The carboniferous Cephalopoda do not depart so widely from the living type (the
Nautilus) as do the more ancient Silurian representatives of the same order; yet
they offer some remarkable forms. Among these is Orthoceras, a siphuncled and
chambered shell, like a Nautilus uncoiled and straightened (Figure 489). Some
species of this genus are several feet long. The Goniatite is another genus,
nearly allied to the Ammonite, from which it differs in having the lobes of the
septa free from lateral denticulations, or crenatures; so that the outline of
these is angular, continuous, and uninterrupted. The species represented in
Figure 490 is found in most localities, and presents the zigzag character of the
septal lobes in perfection. The dorsal position of the siphuncle, however,
clearly distinguishes the Goniatite from the Nautilus, and proves it to have
belonged to the family of the Ammonites, from which, indeed, some authors do not
believe it to be generically distinct.
FOSSIL FISH.
(FIGURE 491. Psammodus porosus, Agassiz. Bone-bed, Mountain Limestone. Bristol,
Armagh.)
(FIGURE 492. Cochliodus contortus, Agassiz. Bone-bed, Mountain Limestone.
Bristol, Armagh.)
The distribution of these is singularly partial; so much so, that M. De Koninck
of Liege, the eminent palaeontologist, once stated to me that, in making his
extensive collection of the fossils of the Mountain Limestone of Belgium, he had
found no more than four or five examples of the bones or teeth of fishes.
Judging from Belgian data, he might have concluded that this class of vertebrata
was of extreme rarity in the Carboniferous seas; whereas the investigation of
other countries has led to quite a different result. Thus, near Clifton, on the
Avon, as well as at numerous places around the Bristol basin from the Mendip
Hills to Tortworth, there is a celebrated "bone-bed," almost entirely made up of
ichthyolites. It occurs at the base of the Lower Limestone shales immediately
resting upon the passage beds of the Old Red Sandstone. Similar bone-beds occur
in the Carboniferous Limestone of Armagh, in Ireland, where they are made up
chiefly of the teeth of fishes of the Placoid order, nearly all of them rolled
as if drifted from a distance. Some teeth are sharp and pointed, as in ordinary
sharks, of which the genus Cladodus afford an illustration; but the majority, as
in Psammodus and Cochliodus, are, like the teeth of the Cestracion of Port
Jackson (see Figure 261), massive palatal teeth fitted for grinding. (See
Figures 491, 492.)
There are upward of seventy other species of fossil fish known in the Mountain
Limestone of the British Islands. The defensive fin-bones of these creatures are
not infrequent at Armagh and Bristol; those known as Oracanthus, Ctenocanthus,
and Onchus are often of a very large size. Ganoid fish, such as Holoptychius,
also occur; but these are far less numerous. The great Megalichthys Hibberti
appears to range from the Upper Coal-measures to the lowest Carboniferous
strata.
FORAMINIFERA.
(FIGURE 493. Fusulina cylindrica, d'Orbigny. Magnified 3 diameters. Mountain
Limestone.)
In the upper part of the Mountain Limestone group in the south-west of England,
near Bristol, limestones having a distinct oolitic structure alternate with
shales. In these rocks the nucleus of every minute spherule is seen, under the
microscope, to consist of a small rhizopod or foraminifer. This division of the
lower animals, which is represented so fully at later epochs by the Nummulites
and their numerous minute allies, appears in the Mountain Limestone to be
restricted to a very few species, among which Textularia, Nodosaria, Endothyra,
and Fusulina (Figure 493), have been recognised. The first two genera are common
to this and all the after periods; the third has been found in the Upper
Silurian, but is not known above the Carboniferous strata; the fourth (Figure
493) is characteristic of the Mountain Limestone in the United States, Arctic
America, Russia, and Asia Minor, but is also known in the Permian.
CHAPTER XXV.
DEVONIAN OR OLD RED SANDSTONE GROUP.
Classification of the Old Red Sandstone in Scotland and in Devonshire.
Upper Old Red Sandstone in Scotland, with Fish and Plants.
Middle Old Red Sandstone.
Classification of the Ichthyolites of the Old Red, and their Relation to Living
Types.
Lower Old Red Sandstone, with Cephalaspis and Pterygotus.
Marine or Devonian Type of Old Red Sandstone.
Table of Devonian Series.
Upper Devonian Rocks and Fossils.
Middle.
Lower.
Eifel Limestone of Germany.
Devonian of Russia.
Devonian Strata of the United States and Canada.
Devonian Plants and Insects of Canada.
CLASSIFICATION OF THE TWO TYPES OF OLD RED SANDSTONE.
We have seen that the Carboniferous strata are surmounted by the Permian and
Trias, both originally included in England under the name "New Red Sandstone,"
from the prevailing red colour of the strata. Under the coal came other red
sandstones and shales which were distinguished by the title of "Old Red
Sandstone." Afterwards the name of "Devonian" was given by Sir R. Murchison and
Professor Sedgwick to marine fossiliferous strata which, in the south of
England, occupy a similar position between the overlying coal and the underlying
Silurian formations.
It may be truly said that in the British Isles the rocks of this age present
themselves in their mineral aspect, and even to some extent in their fossil
contents, under two very different forms; the one as distinct from the other as
are often lacustrine or fluviatile from marine strata. It has indeed been
suggested that by far the greater part of the deposits belonging to what may be
termed the Old Red Sandstone type are of fresh-water origin. The number of land-
plants, the character of the fishes, and the fact that the only shell yet
discovered belongs to the genus Anodonta, must be allowed to lend no small
countenance to this opinion. In this case the difficulty of classification when
the strata of this type are compared in different regions, even where they are
contiguous, may arise partly from their having been formed in distinct
hydrographical basins, or in the neighbourhood of the land in shallow parts of
the sea into which large bodies of fresh-water entered, and where no marine
mollusca or corals could flourish. Under such geographical conditions the
limited extent of some kinds of sediment, as well as the absence of those marine
forms by which we are able to identify or contrast marine formations, may be
explained, while the great thickness of the rocks, which might seem at first
sight to require a corresponding depth of water, can often be shown to have been
due to the gradual sinking down of the bottom of the estuary or sea where the
sediment was accumulated.
Another active cause of local variation in Scotland was the frequency of
contemporaneous volcanic eruptions; some of the rocks derived from this source,
as between the Grampians and the Tay, having formed islands in the sea, and
having been converted into shingle and conglomerate, before the upper portions
of the red shales and sandstones were superimposed.
The dearth of calcareous matter over wide areas is characteristic of the Old Red
Sandstone. This is, no doubt, in great part due to the absence of shells and
corals; but why should these be so generally wanting in all sedimentary rocks
the colour of which is determined by the red oxide of iron? Some geologists are
of opinion that the waters impregnated with this oxide were prejudicial to
living beings, others that strata permeated with this oxide would not preserve
such fossil remains.
In regard to the two types, the Old Red Sandstone and the Devonian, I shall
first treat of them separately, and then allude to the proofs of their having
been to a great extent contemporaneous. That they constitute a series of rocks
intermediate in date between the lowest Carboniferous and the uppermost Silurian
is not disputed by the ablest geologists; and it can no longer be contended that
the Upper, Middle, and Lower Old Red Sandstone preceded in date the three
divisions to which, by aid of the marine shells, the Devonian rocks have been
referred, while, on the other hand, we have not yet data for enabling us to
affirm to what extent the subdivisions of the one series may be the equivalents
in time of those of the other.
UPPER OLD RED SANDSTONE.
(FIGURE 494. Anodonta Jukesii, Forbes. Upper Devonian, Kiltorkan, Ireland.)
(FIGURE 495. Bifurcating branch of Lepidodendron Griffithsii, Brongn. Upper
Devonian, Kilkenny.)
(FIGURE 496. Palaeopteris Hibernica, Schimp. (Cyclopteris Hibernica), Edward
Forbes (Adiantites, Gop.). Upper Devonian, Kilkenny.)
The highest beds of the series in Scotland, lying immediately below the coal in
Fife, are composed of yellow sandstone well seen at Dura Den, near Coupar, in
Fife, where, although the strata contain no mollusca, fish have been found
abundantly, and have been referred to the genera Holoptychius, Pamphractus,
Glyptopomus, and many others. In the county of Cork, in Ireland, a similar
yellow sandstone occurs containing fish of genera characteristic of the Scotch
Old Red Sandstone, as for example Coccosteus (a form represented by many species
in the Old Red Sandstone and by one only in the Carboniferous group), and
Glytolepis and Asterolepis, both exclusively confined to the "Old Red." In the
same Irish sandstone at Kiltorkan has been found an Anodonta or fresh-water
mussel, the only shell hitherto discovered in the Old Red Sandstone of the
British Isles (see Figure 494). In the same formation are found the fern (Figure
496) and the Lepidodendron (Figure 495), and other species of plants, some of
which, Professor Heer remarks, agree specifically with species from the lower
carboniferous beds. This induces him to lean to the opinion long ago advocated
by Sir Richard Griffiths, that the yellow sandstone, in spite of its fish
remains, should be classed as Lower Carboniferous, an opinion which I am not yet
prepared to adopt. Between the Mountain Limestone and the yellow sandstone in
the south-west of Ireland there intervenes a formation no less than 5000 feet
thick, called the "Carboniferous slate," and at the base of this, in some
places, are local deposits, such as the Glengariff Grits, which appear to be
beds of passage between the Carboniferous and Old Red Sandstone groups.
It is a remarkable result of the recent examination of the fossil flora of Bear
Island, latitude 74 degrees 30' N., that Professor Heer has described as
occurring in that part of the Arctic region (nearly twenty-six degrees to the
north of the Irish locality) a flora agreeing in several of its species with
that of the yellow sandstones of Ireland. This Bear Island flora is believed by
Professor Heer to comprise species of plants some of which ascend even to the
higher stages of the European Carboniferous formation, or as high as the
Mountain Limestone and Millstone Grit. Palaeontologists have long maintained
that the same species which have a wide range in space are also the most
persistent in time, which may prepare us to find that some plants having a vast
geographical range may also have endured from the period of the Upper Devonian
to that of the Millstone Grit.
(FIGURE 497. Scale of Holoptychius nobilissimus, Agassiz. Clashbinnie. 1/2
natural size.)
(FIGURE 498. Holoptychius, as restored by Professor Huxley.
a. The fringed pectoral fins.
b. The fringed ventral fins.
c. Anal fin.
d, e. Dorsal fins.)
Outliers of the Upper "Old Red" occur unconformably on older members of the
group, and the formation represented at Whiteness, near Arbroath, a, Figure 55,
may probably be one of these outliers, though the want of organic remains
renders this uncertain. It is not improbable that the beds given in this section
as Nos. 1, 2, and 3, may all belong to the early part of the period of the Upper
Old Red, as some scales of Holoptychius nobilissimus have been found scattered
through these beds, No. 2, in Strathmore. Another nearly allied Holoptychius
occurs in Dura Den, see Figure 498 of this fish and also Figure 497 of one of
its scales, as these last are often the only parts met with; being scattered in
Forfarshire through red-coloured shales and sandstones, as are scales of a large
species of the same genus in a corresponding matrix in Herefordshire. (Siluria
4th edition page 265.) The number of fish obtained from the British Upper Old
Red Sandstone amounts to fifteen species referred to eleven genera.
Sir R. Murchison groups with this upper division of the Old Red of Scotland
certain light-red and yellow sandstones and grits which occur in the
northernmost part of the mainland, and extend also into the Orkney and Shetland
Islands. They contain Calamites and other plants which agree generically with
Carboniferous forms.
MIDDLE OLD RED SANDSTONE.
In the northern part of Scotland there occur a great series of bituminous
schists and flagstones, to the fossil fish of which attention was first called
by the late Hugh Miller. They were afterwards described by Agassiz, and the
rocks containing them were examined by Sir R. Murchison and Professor Sedgwick,
in Caithness, Cromarty, Moray, Nairn, Gamrie in Banff, and the Orkneys and
Shetlands, in which great numbers of fossil fish have been found. These were at
first supposed to be the oldest known vertebrate animals, as in Cromarty the
beds in which they occur seem to form the base of the Old Red system resting
almost immediately on the crystalline or metamorphic rocks. But in fact these
fish-bearing beds, when they are traced from north to south, or to the central
parts of Scotland, thin out, so that their relative age to the Lower Old Red
Sandstone, presently to be mentioned, was not at first detected, the two
formations not appearing in superposition in the same district. In Caithness,
however, many hundred feet below the fish-zone of the middle division, remains
of Pteraspis were found by Mr. Peach in 1861. This genus has never yet been
found in either of the two higher divisions of the Old Red Sandstone, and
confirms Sir R. Murchison's previous suspicion that the rocks in which it occurs
belong to the Lower "Old Red," or agree in age with the Arbroath paving-stone.
(Siluria 4th edition page 258.)
FOSSIL FISH OF THE MIDDLE OLD RED SANDSTONE.
The Devonian fish were referred by Agassiz to two of his great orders, namely,
the Placoids and Ganoids. Of the first of these, which in the Recent period
comprise the shark, the dog-fish, and the ray, no entire skeletons are
preserved, but fin-spines, called ichthyodorulites, and teeth occur. On such
remains the genera Onchus, Odontacanthus, and Ctenodus, a supposed cestraciont,
and some others, have been established.
(FIGURE 499. Polypterus. See Agassiz, "Recherces sur les Poissons Fossiles."
Living in the Nile and other African rivers.
a. One of the fringed pectoral fins.
b. One of the ventral fins.
c. Anal fin.
d. Dorsal fin, or row of finlets.)
(FIGURE 500. Restoration of Osteolepis. Pander. Old Red Sandstone, or Devonian.
a. One of the fringed pectoral fins.
b. One of the ventral fins.
c. Anal fin.
d, e. Dorsal fins.)
By far the greater number of the Old Red Sandstone fishes belong to a sub-order
of Ganoids instituted by Huxley in 1861, and for which he has proposed the name
of Crossopterygidae (Abridged from crossotos, a fringe, and pteryx, a fin.), or
the fringe-finned, in consideration of the peculiar manner in which the fin-rays
of the paired fins are arranged so as to form a fringe round a central lobe, as
in the Polypterus (see a, Figure 499), a genus of which there are several
species now inhabiting the Nile and other African rivers. The reader will at
once recognise in Osteolepis (Figure 500), one of the common fishes of the Old
Red Sandstone, many points of analogy with Polypterus. They not only agree in
the structure of the fin, at first pointed out by Huxley, but also in the
position of the pectoral, ventral, and anal fins, and in having an elongated
body and rhomboidal scales. On the other hand, the tail is more symmetrical in
the recent fish, which has also an apparatus of dorsal finlets of a very
abnormal character, both as to number and structure. As to the dorsals of
Osteolepis, they are regular in structure and position, having nothing
remarkable about them, except that there are two of them, which is comparatively
unusual in living fish.
Among the "fringe-finned" Ganoids we find some with rhomboidal scales, such as
Osteolepis, Figure 500; others with cycloidal scales, as Holoptychius, before
mentioned (see Figure 498). In the genera Dipterus and Diplopterus, as Hugh
Miller pointed out, and in several other of the fringe-finned genera, as in
Gyroptychius and Glyptolepis, the two dorsals are placed far backward, or
directly over the ventral and anal fins. The Asterolepis was a ganoid fish of
gigantic dimensions. A. Asmusii, Eichwald, a species characteristic of the Old
Red Sandstone of Russia, as well as that of Scotland, attained the length of
between twenty and thirty feet. It was clothed with strong bony armour, embossed
with star-like tubercles, but it had only a cartilaginous skeleton. The mouth
was furnished with two rows of teeth, the outer ones small and fish-like, the
inner larger and with a reptilian character. The Asterolepis occurs also in the
Devonian rocks of North America.
If we except the Placoids already alluded to, and a few other families of
doubtful affinities, all the Old Red Sandstone fishes are Ganoids, an order so
named by Agassiz from the shining outer surface of their scales; but Professor
Huxley has also called our attention to the fact that, while a few of the
primary and the great majority of the secondary Ganoids resemble the living bony
pike, Lepidosteus, or the Amia, genera now found in North American rivers, and
one of them, Lepidosteus, extending as far south as Guatemala, the
Crossopterygii, or fringe-finned Ichthyolites, of the Old Red are closely
related to the African Polypterus, which is represented by five or six species
now inhabiting the Nile and the rivers of Senegal. These North American and
African Ganoids are quite exceptional in the living creation; they are entirely
confined to the northern hemisphere, unless some species of Polypterus range to
the south of the line in Africa; and, out of about 9000 living species of fish
known to M. Gunther, and of which more than 6000 are now preserved in the
British Museum, they probably constitute no more than nine.
If many circumstances favour the theory of the fresh-water origin of the Old Red
Sandstone, this view of its nature is not a little confirmed by our finding that
it is in Llake Superior and the other inland Canadian seas of fresh water, and
in the Mississippi and African rivers, that we at present find those fish which
have the nearest affinity to the fossil forms of this ancient formation.
(FIGURE 501. Pterichthys, Agassiz; Upper side, showing mouth; as restored by H.
Miller.)
Among the anomalous forms of Old Red fishes not referable to Huxley's
Crossopterygii is the Pterichthys, of which five species have been found in the
middle division of the Old Red of Scotland. Some writers have compared their
shelly covering to that of Crustaceans, with which, however, they have no real
affinity. The wing-like appendages, whence the genus is named, were first
supposed by Hugh Miller to be paddles, like those of the turtle; and there can
now be no doubt that they do really correspond with the pectoral fins.
The number of species of fish already obtained from the middle division of the
Old Red Sandstone in Great Britain is about 70, and the principal genera,
besides Osteolepis and Pterichthys, already mentioned, are Glyptolepis,
Diplacanthus, Dendrodus, Coccosteus, Cheirancanthus, and Acanthoides.
LOWER OLD RED SANDSTONE.
(FIGURE 502. Cephalaspis Lyellii, Agassiz. Length 6 3/4 inches. From a specimen
in my collection found at Glammiss, in Forfarshire. (See other figures, Agassiz,
volume 2 table 1 a and 1 b.
a. One of the peculiar scales with which the head is covered when perfect. These
scales are generally removed, as in the specimen above figured.
b, c. Scales from different parts of the body and tail.)
The third or lowest division south of the Grampians consists of grey paving-
stone and roofing-slate, with associated red and grey shales; these strata
underlie a dense mass of conglomerate. In these grey beds several remarkable
fish have been found of the genus named by Agassiz Cephalaspis, or "buckler-
headed," from the extraordinary shield which covers the head (see Figure 502),
and which has often been mistaken for that of a trilobite, such as Asaphus. A
species of Pteraspis, of the same family, has also been found by the Reverend
Hugh Mitchell in beds of corresponding age in Perthshire; and Mr. Powrie
enumerates no less than five genera of the family Acanthodidae, the spines,
scales, and other remains of which have been detected in the grey flaggy
sandstones. (Powrie Geological Quarterly Journal volume 20 page 417.)
(FIGURE 503. Pterygotus anglicus, Agassiz. Middle portion of the back of the
head called the seraphim.)
(FIGURE 504. Pterygotus anglicus, Agassiz. Forfarshire. Ventral aspect. Restored
by H. Wodward, F.G.S.
a. Carapace, showing the large sessile eyes at the anterior angles.
b. The metastoma or post-oral plate (serving the office of a lower lip).
c, c. Chelate appendages (antennules).
d. First pair of simple palpi (antennae).
e. Second pair of simple palpi (mandibles).
f. Third pair of simple palpi (first maxillae).
g. Pair of swimming feet with their broad basal joints, whose serrated edges
serve the office of maxillae.
h. Thoracic plate covering the first two thoracic segments, which are indicated
by the figures 1, 2, and a dotted line.
1-6. Thoracic segments.
7-12. Abdominal segments.
13. Telson, or tail-plate.)
In the same formation at Carmylie, in Forfarshire, commonly known as the
Arbroath paving-stone, fragments of a huge crustacean have been met with from
time to time. They are called by the Scotch quarrymen the "Seraphim," from the
wing-like form and feather-like ornament of the thoracic appendage, the part
most usually met with. Agassiz, having previously referred some of these
fragments to the class of fishes, was the first to recognise their crustacean
character, and, although at the time unable correctly to determine the true
relation of the several parts, he figured the portions on which he founded his
opinion, in the first plate of his "Poissons Fossiles du Vieux Gres Rouge."
A restoration in correct proportion to the size of the fragments of P. anglicus
(Figure 504), from the Lower Old Red Sandstone of Perthshire and Forfarshire,
would give us a creature measuring from five to six feet in length, and more
than one foot across.
The largest crustaceans living at the present day are the Inachus Kaempferi, of
De Haan, from Japan (a brachyurous or short-tailed crab), chiefly remarkable for
the extraordinary length of its limbs; the fore-arm measuring four feet in
length, and the others in proportion, so that it covers about 25 square feet of
ground; and the Limulus Moluccanus, the great King Crab of China and the Eastern
seas, which, when adult, measures 1 1/2 foot across its carapace, and is three
feet in length.
(FIGURE 505. Parka decipiens, Fleming. In sandstone of lower beds of Old Red,
Ley's Mill, Forfarshire.)
(FIGURE 506. Parka decipiens, Fleming. In shale of Lower Old Red, Park Hill,
Fife.)
(FIGURE 507. Shale of Old Red Sandstone. Forfarshire. With impression of plants
and eggs of Crustaceans.
a. Two pair of ova? resembling those of large Salamanders or Tritons-- on the
same leaf.
b, b. Detached ova.)
Besides some species of Pterygotus, several of the allied genus Eurypterus occur
in the Lower Old Red Sandstone, and with them the remains of grass-like plants
so abundant in Forfarshire and Kincardineshire as to be useful to the geologist
by enabling him to identify the inferior strata at distant points. Some
botanists have suggested that these plants may be of the family Fluviales, and
of fresh-water genera. They are accompanied by fossils, called "berries" by the
quarrymen, which they compared to a compressed blackberry (see Figures 505,
506), and which were called "Parka" by Dr. Fleming. They are now considered by
Mr. Powrie to be the eggs of crustaceans, which is highly probable, for they
have not only been found with Pterygotus anglicus in Forfarshire and Perthshire,
but also in the Upper Silurian strata of England, in which species of the same
genus, Pterygotus, occur.
The grandest exhibitions, says Sir R. Murchison, of the Old Red Sandstone in
England and Wales appear in the escarpments of the Black Mountains and in the
Fans of Brecon and Carmarthen, the one 2862, and the other 2590 feet above the
sea. The mass of red and brown sandstone in these mountains is estimated at not
less than 10,000 feet, clearly intercalated between the Carboniferous and
Silurian strata. No shells or corals have ever been found in the whole series,
not even where the beds are calcareous, forming irregular courses of
concretionary lumps called "corn-stones," which may be described as mottled red
and green earthy limestones. The fishes of this lowest English Old Red are
Cephalaspis and Pteraspis, specifically different from species of the same
genera which occur in the uppermost Ludlow or Silurian tilestones. Crustaceans
also of the genus Eurypterus are met with.
MARINE OR DEVONIAN TYPE.
We may now speak of the marine type of the British strata intermediate between
the Carboniferous and Silurian, in treating of which we shall find it much more
easy to identify the Upper, Middle, and Lower divisions with strata of the same
age in other countries. It was not until the year 1836 that Sir R. Murchison and
Professor Sedgwick discovered that the culmiferous or anthracitic shales and
sandstones of North Devon, several thousand feet thick, belonged to the coal,
and that the beds below them, which are of still greater thickness, and which,
like the carboniferous strata, had been confounded under the general name
"graywacke," occupied a geological position corresponding to that of the Old Red
Sandstone already described. In this reform they were aided by a suggestion of
Mr. Lonsdale, who, after studying the Devonshire fossils, perceived that they
belonged to a peculiar palaeontological type of intermediate character between
the Carboniferous and Silurian.
It is in the north of Devon that these formations may best be studied, where
they have been divided into an Upper, Middle, and Lower Group, and where,
although much contorted and folded, they have for the most part escaped being
altered by intrusive trap-rocks and by granite, which in Dartmoor and the more
southern parts of the same county have often reduced them to a crystalline or
metamorphic state.
TABLE 25.1 DEVONIAN SERIES IN NORTH DEVON.
UPPER DEVONIAN OR PILTON GROUP.
a. Sandy slates and schists with fossils, 36 species out of 110 common to the
Carboniferous group (Pilton, Barnstaple, etc.), resting on soft schists in which
fossils are very abundant (Croyde, etc.), and which pass down into
b. Yellow, brown, and red sandstone, with land plants (Cyclopteris, etc.) and
marine shells. One zone, characterised by the abundance of cucullaea (Baggy
Point, Marwood, Sloly, etc.) resting on hard grey and reddish sandstone and
micaceous flags, no fossils yet found (Dulverton, Pickwell, Down, etc.)
MIDDLE DEVONIAN OR ILFRACOMBE GROUP.
a. Green glossy slates of considerable thickness, no fossils yet recorded from
these beds (Mortenoe, Lee Bay, etc.).
b. Slates and schists, with several irregular courses of limestone containing
shells and corals like those of the Plymouth Limestone (Combe Martin,
Ilfracombe, etc.).
LOWER DEVONIAN OR LYNTON GROUP.
a. Hard, greenish, red, and purple sandstone-- no fossils yet found (Hangman
Hill, etc.).
b. Soft slates with subordinate sandstones-- fossils numerous at various
horizons-- Orthis, Corals, Encrinites, etc. (Valley of Rocks, Lynmouth, etc.).
Table 25.1 exhibits the sequence of the strata or subdivisions as seen both on
the sea-coast of the British Channel and in the interior of Devon. It will be
seen that in all main points it agrees with the table drawn up in 1864 for the
sixth edition of my "Elements." Mr. Etheridge has since published an excellent
account of the different subdivisions of the rocks and their fossils, and has
also pointed out their relation to the corresponding marine strata of the
Continent. (Quarterly Geological Journal volume 23 1867.) The slight
modifications introduced in my table since 1864 are the result of a tour made in
1870 in company with Mr. T. Mck. Hughes, when we had the advantage of Mr.
Etheridge's memoir as our guide.
The place of the sandstones of the Foreland is not yet clearly made out, as they
are cut off by a great fault and disturbance.
UPPER DEVONIAN ROCKS.
(FIGURE 508. Spirifera disjuncta, Sowerby. Syn. Sp. Verneuilii, Murch. Upper
Devonian, Boulogne.)
(FIGURE 509. Phacops latifrons, Bronn. Characteristic of the Devonian in Europe,
Asia, and N. and S. America.)
(FIGURE 510. Clymenia linearis, Munster. Petherwyn, Cornwall; Elbersreuth,
Bavaria.)
(FIGURE 511. Cypridina serrato-striata, Sandberger, Weilburg, etc.; Cornwall,
Nassau, Saxony, Belgium.)
The slates and sandstones of Barnstaple (a and b of the preceding section)
contain the shell Spirifera disjuncta, Sowerby (S. Verneuilii, Murch.), (see
Figure 508), which has a very wide range in Europe, Asia Minor, and even China;
also Strophalosia caperata, together with the large trilobite Phacops latifrons,
Bronn. (See Figure 509), which is all but world-wide in its distribution. The
fossils are numerous, and comprise about 150 species of mollusca, a fifth of
which pass up into the overlying Carboniferous rocks. To this Upper Devonian
belong a series of limestones and slates well developed at Petherwyn, in
Cornwall, where they have yielded 75 species of fossils. The genus of
Cephalopoda called Clymenia (Figure 510) is represented by no less than eleven
species, and strata occupying the same position in Germany are called Clymenien-
Kalk, or sometimes Cypridinen-Schiefer, on account of the number of minute
bivalve shells of the crustacean called Cypridina serrato-striata (Figure 511),
which is found in these beds, in the Rhenish provinces, the Harz, Saxony, and
Silesia, as well as in Cornwall and Belgium.
MIDDLE DEVONIAN ROCKS.
(FIGURE 512. Heliolites porosa, Goldf. sp. (Porites pyriformis, Lonsd.)
a. Portion of the same magnified. Middle Devonian, Torquay, Plymouth; Eifel.)
(FIGURE 513. Favosites cervicornis, Blainv. S. Devon, from a polished specimen.
a. Portion of the same magnified, to show the pores.)
(FIGURE 514. Cyathophyllum caespitosum, Goldf.; Plymouth and Ilfracombe.
b. A terminal star.
c. Vertical section, exhibiting transverse plates, and part of another branch.)
We come next to the most typical portion of the Devonian system, including the
great limestones of Plymouth and Torbay, replete with shells, trilobites, and
corals. Of the corals 51 species are enumerated by Mr. Etheridge, none of which
pass into the Carboniferous formation. Among the genera we find Favosites,
Heliolites, and Cyathophyllum. The two former genera are very frequent in
Silurian rocks: some few even of the species are said to be common to the
Devonian and Silurian groups, as, for example, Favosites cervicornis (Figure
513), one of the commonest of all the Devonshire fossils. The Cyathophyllum
caespitosum (Figure 514) and Heliolites pyriformis (Figure 512) are species
peculiar to this formation.
(FIGURE 515. Stringocephalus Burtini, Def.
a. Valves united.
b. Interior of ventral or large valve, showing thick partition and portion of a
large process which projects from the dorsal valve across the shell.)
(FIGURE 516. Uncites Gryphus, Def. Middle Devonian. S. Devon and the Continent.)
With the above are found no less than eleven genera of stone-lilies or crinoids,
some of them, such as Cupressocrinites, distinct from any Carboniferous forms.
The mollusks, also, are no less characteristic; of 68 species of Brachiopoda,
ten only are common to the Carboniferous Limestone. The Stringocephalus Burtini
(Figure 515) and Uncites Gryphus (Figure 516) may be mentioned as exclusively
Middle Devonian genera, and extremely characteristic of the same division in
Belgium. The Stringocephalus is also so abundant in the Middle Devonian of the
banks of the Rhine as to have suggested the name of Stringocephalus Limestone.
The only two species of Brachiopoda common to the Silurian and Devonian
formations are Atrypa reticularis (Figure 532), which seems to have been a
cosmopolite species, and Strophomena rhomboidalis.
(FIGURE 517. Megalodon cucullatus, Sowerby. Eifel; also Bradley, S. Devon.
a. The valves united.
b. Interior of valve, showing the large cardinal tooth.)
(FIGURE 518. Conularia ornata, D'Arch. and De Vern. (Geological Transactions
Sec. Ser. volume 6. Plate 29.) Refrath, near Cologne.)
(FIGURE 519. Bronteus flabellifer, Goldf. Mid. Devon; S. Devon; and the Eifel.)
Among the peculiar lamellibranchiate bivalves common to the Plymouth limestone
of Devonshire and the Continent, we find the Megalodon (Figure 517). There are
also twelve genera of Gasteropods which have yielded 36 species, four of which
pass to the Carboniferous group, namely Macrocheilus, Acroculia, Euomphalus, and
Murchisonia. Pteropods occur, such as Conularia (Figure 518), and Cephalopods,
such as Cyrtoceras, Gyroceras, Orthoceras, and others, nearly all of genera
distinct from those prevailing in the Upper Devonian Limestone, or Clymenien-
kalk of the Germans already mentioned. Although but few species of Trilobites
occur, the characteristic Bronteus flabellifer (Figure 519) is far from rare,
and all collectors are familiar with its fan-like tail. In this same group,
called, as before stated, the Stringocephalus, or Eifel Limestone, in Germany,
several fish remains have been detected, and among others the remarkable genus
Coccosteus, covered with its tuberculated bony armour; and these ichthyolites
serve, as Sir R. Murchison observes (Siluria page 362), to identify this middle
marine Devonian with the Old Red Sandstone of Britain and Russia.
(FIGURE 520. Calceola sandalina, Lam. Eifel; also South Devon.
a. Ventral valve.
b. Inner side of dorsal valve.)
Beneath the Eifel Limestone (the great central and typical member of "the
Devonian" on the Continent) lie certain schists called by German writers
"Calceola-schiefer," because they contain in abundance a fossil body of very
curious structure, Calceola sandalina (Figure 520), which has been usually
considered a brachiopod, but which some naturalists have lately referred to a
Goniophyllum, supposing it to be an abnormal form of the order Zoantharia rugosa
(see Figure 474), differing from all other corals in being furnished with a
strong operculum. This is by no means a rare fossil in the slaty limestone of
South Devon, and, like the Eifel form, is confined to the middle group of this
country.
LOWER DEVONIAN ROCKS.
(FIGURE 521. Spirifera mucronata, Hall. Devonian of Pennsylvania.)
A great series of sandstones and glossy slates, with Crinoids, Brachiopods, and
some corals, occurring on the coast at Lynmouth and the neighbourhood, and
called the Lynton Group (see Table 25.1), form the lowest member of the Devonian
in North Devon. Among the 18 species of all classes enumerated by Mr. Etheridge,
two-thirds are common to the Middle Devonian, but only one, the ubiquitous
Atrypa reticularis, can with certainty be identified with Silurian species.
Among the characteristic forms are Alveolites suborbicularis, also common to
this formation in the Rhine, and Orthis arcuata, very widely spread in the North
Devon localities. But we may expect a large addition to the number of fossils
whenever these strata shall have been carefully searched. The Spirifer Sandstone
of Sandberger, as exhibited in the rocks bordering the Rhine between Coblentz
and Caub, belong to this Lower division, and the same broad-winged Spirifers
distinguish the Devonian strata of North America.
(FIGURE 522. Homalonotus armatus, Burmeister. Lower Devonian; Daun, in the
Eifel; and S. Devon.
Obs. The two rows of spines down the body give an appearance of more distinct
trilobation than really occurs in this or most other species of the genus.)
Among the Trilobites of this era several large species of Homalonotus (Figure
522) are conspicuous. The genus is still better known as a Silurian form, but
the spinose species appear to belong exclusively to the "Lower Devonian," and
are found in Britain, Europe, and the Cape of Good Hope.
DEVONIAN OF RUSSIA.
The Devonian strata of Russia extend, according to Sir R. Murchison, over a
region more spacious than the British Isles; and it is remarkable that, where
they consist of sandstone like the "Old Red" of Scotland and Central England,
they are tenanted by fossil fishes often of the same species and still oftener
of the same genera as the British, whereas when they consist of limestone they
contain shells similar to those of Devonshire, thus confirming, as Sir Roderick
has pointed out, the contemporaneous origin which had been previously assigned
to formations exhibiting two very distinct mineral types in different parts of
Britain. (Murchison's Siluria page 329.) The calcareous and the arenaceous rocks
of Russia above alluded to alternate in such a manner as to leave no doubt of
their having been deposited in different parts of the same great period.
DEVONIAN STRATA IN THE UNITED STATES AND CANADA.
(FIGURE 523. Psilophyton princeps, Dawson, Quarterly Geological Journal volume
15 1863; and Canada Survey 1863. Species characteristic of the whole Devonian
series in North America.
a. Fruit; natural size.
b. Stem; natural size.
c. Scalariform tissue of the axis highly magnified.)
Between the Carboniferous and Silurian strata there intervenes, in the United
States and Canada, a great series of formations referable to the Devonian group,
comprising some strata of marine origin abounding in shells and corals, and
others of shallow-water and littoral origin in which terrestrial plants abound.
The fossils, both of the deep and shallow water strata, are very analogous to
those of Europe, the species being in some cases the same. In Eastern Canada Sir
W. Logan has pointed out that in the peninsula of Gaspe, south of the estuary of
St. Lawrence, a mass of sandstone, conglomerate, and shale referable to this
period occurs, rich in vegetable remains, together with some fish-spines. Far
down in the sandstones of Gaspe, Dr. Dawson found, in 1869, an entire specimen
of the genus Cephalaspis, a form so characteristic, as we have already seen, of
the Scotch Lower Old Red Sandstone. Some of the sandstones are ripple-marked,
and towards the upper part of the whole series a thin seam of coal has been
observed, measuring, together with some associated carbonaceous shale, about
three inches in thickness. It rests on an under-clay in which are the roots of
Psilophyton (see Figure 523). At many other levels rootlets of this same plant
have been shown by Principal Dawson to penetrate the clays, and to play the same
part as do the rootlets of Stigmaria in the coal formation.
We had already learnt from the works of Goppert, Unger, and Bronn that the
European plants of the Devonian epoch resemble generically, with few exceptions,
those already known as Carboniferous; and Dr. Dawson, in 1859, enumerated 32
genera and 69 species which he had then obtained from the State of New York and
Canada. A perusal of his catalogue (Quarterly Geological Journal volume 15 page
477 1859; also volume 18 page 296 1862.), comprising Coniferae, Sigillariae,
Calamites, Asterophyllites, Lepidodendra, and ferns of the genera Cyclopteris,
Neuropteris, Sphenopteris, and others, together with fruits, such as
Cardiocarpum and Trigonocarpum, might dispose geologists to believe that they
were presented with a list of Carboniferous fossils, the difference of the
species from those of the coal-measures, and even a slight admixture of genera
unknown in Europe, being naturally ascribed to geographical distribution and the
distance of the New from the Old World. But fortunately the coal formation is
fully developed on the other side of the Atlantic, and is singularly like that
of Europe, both lithologically and in the species of its fossil plants. There is
also the most unequivocal evidence of relative age afforded by superposition,
for the Devonian strata in the United States are seen to crop out from beneath
the Carboniferous on the borders of Pennsylvania and New York, where both
formations are of great thickness.
The number of American Devonian plants has now been raised by Dr. Dawson to 120,
to which we may add about 80 from the European flora of the same age, so that
already the vegetation of this period is beginning to be nearly half as rich as
that of the coal-measures which have been studied for so much longer a time and
over so much wider an area. The Psilophyton above alluded to is believed by Dr.
Dawson to be a lycopodiaceous plant, branching dichotomously (see P. princeps,
Figure 523), with stems springing from a rhizome, which last has circular
areoles, much resembling those of Stigmaria, and like it sending forth
cylindrical rootlets. The extreme points of some of the branchlets are rolled up
so as to resemble the croziers or circinate vernation of ferns; the leaves or
bracts, a, supposed to belong to the same plant, are described by Dawson as
having inclosed the fructification. The remains of Psilophyton princeps have
been traced through all the members of the Devonian series in America, and Dr.
Dawson has lately recognised it in specimens of Old Red Sandstone from the north
of Scotland.
The monotonous character of the Carboniferous flora might be explained by
imagining that we have only the vegetation handed down to us of one set of
stations, consisting of wide swampy flats. But Dr. Dawson supposes that the
geographical conditions under which the Devonian plants grew were more varied,
and had more of an upland character. If so, the limitation of this more ancient
flora, represented by so many genera and species, to the gymnospermous and
cryptogamous orders, and the absence or extreme rarity of plants of higher
grade, lead us naturally to speculate on the theory of progressive development,
however difficult it may be to avail ourselves of this explanation, so long as
we meet with even a few exceptional cases of what may seem to be
monocotyledonous or dicotyledonous exogens.
DEVONIAN INSECTS OF CANADA.
The earliest known insects were brought to light in 1865 in the Devonian strata
of St. John's, New Brunswick, and are referred by Mr. Scudder to four species of
Neuroptera. One of them is a gigantic Ephemera, and measured five inches in
expanse of wing.
Like many other ancient animals, says Dr. Dawson, they show a remarkable union
of characters now found in distinct orders of insects, or constitute what have
been named "synthetic types." Of this kind is a stridulating or musical
apparatus like that of the cricket in an insect otherwise allied to the
Neuroptera. This structure, as Dr. Dawson observes, if rightly interpreted by
Mr. Scudder, introduces us to the sounds of the Devonian woods, bringing before
our imagination the trill and hum of insect life that enlivened the solitudes of
these strange old forests.
CHAPTER XXVI.
SILURIAN GROUP.
Classification of the Silurian Rocks.
Ludlow Formation and Fossils.
Bone-bed of the Upper Ludlow.
Lower Ludlow Shales with Pentamerus.
Oldest known Remains of fossil Fish.
Table of the progressive Discovery of Vertebrata in older Rocks.
Wenlock Formation, Corals, Cystideans and Trilobites.
Llandovery Group or Beds of Passage.
Lower Silurian Rocks.
Caradoc and Bala Beds.
Brachiopoda.
Trilobites.
Cystideae.
Graptolites.
Llandeilo Flags.
Arenig or Stiper-stones Group.
Foreign Silurian Equivalents in Europe.
Silurian Strata of the United States.
Canadian Equivalents.
Amount of specific Agreement of Fossils with those of Europe.
CLASSIFICATION OF THE SILURIAN ROCKS.
We come next in descending order to that division of Primary or Palaeozoic rocks
which immediately underlie the Devonian group or Old Red Sandstone. For these
strata Sir Roderick Murchison first proposed the name of Silurian when he had
studied and classified them in that part of Wales and some of the contiguous
counties of England which once constituted the kingdom of the Silures, a tribe
of ancient Britons. Table 26.1 will explain the two principal divisions, Upper
and Lower, of the Silurian rocks, and the minor subdivisions usually adopted,
comprehending all the strata originally embraced in the Silurian system by Sir
Roderick Murchison. The formations below the Arenig or Stiper-stones group are
treated of in the next chapter, when the "Primordial" or Cambrian group is
described.
TABLE 26.1. SILURIAN ROCKS (THICKNESS GIVEN IN FEET).
UPPER SILURIAN ROCKS.
1. LUDLOW FORMATION:
a. Upper Ludlow beds: 780.
b. Lower Ludlow beds: 1,050.
2. WENLOCK FORMATION:
a. Wenlock limestone and shale and
b. Woolhope limestone and shale, and Denbighshire grits: above 4,000.
3. LLANDOVERY FORMATION (Beds of passage between Upper and Lower Silurian):
a. Upper Llandovery (May-Hill beds): 800.
b. Lower Llandovery: 600-1,000.
LOWER SILURIAN ROCKS.
1. BALA AND CARADOC BEDS, including volcanic rocks: 12,000.
2. LLANDEILO FLAGS, including volcanic rocks: 4,500.
3. ARENIG OR STIPER-STONES GROUP, including volcanic rocks: above 10,000.
UPPER SILURIAN ROCKS.
1. LUDLOW FORMATION.
This member of the Upper Silurian group, as will be seen by Table 26.1, is of
great thickness, and subdivided into two parts-- the Upper Ludlow and the Lower
Ludlow. Each of these may be distinguished near the town of Ludlow, and at other
places in Shropshire and Herefordshire, by peculiar organic remains; but out of
more than 500 species found in the Ludlow formation as a whole, not more than
five species per hundred are common to the overlying Devonian. The student may
refer to the excellent tables given in the last edition of Sir R. Murchison's
Siluria for a list of the organic remains of all classes distributed through the
different subdivisions of the Upper and Lower Silurian.
A. UPPER LUDLOW: DOWNTON SANDSTONE.
At the top of this subdivision there occur beds of fine-grained yellowish
sandstone and hard reddish grits which were formerly referred by Sir R.
Murchison to the Old Red Sandstone, under the name of "Tilestones." In mineral
character this group forms a transition from the Silurian to the Old Red
Sandstone, the strata of both being conformable; but it is now ascertained that
the fossils agree in great part specifically, and in general character entirely,
with those of the underlying Upper Ludlow rocks. Among these are Orthoceras
bullatum, Platyschisma helicites, Bellerophon trilobatus, Chonetes lata, etc.,
with numerous defenses of fishes.
These beds, therefore, now generally called the "Downton Sandstone," are classed
as the newest member of the Upper Silurian. They are well seen at Downton
Castle, near Ludlow, where they are quarried for building, and at Kington, in
Herefordshire. In the latter place, as well as at Ludlow, crustaceans of the
genera Pterygotus (for genus see Figure 504) and Eurypterus are met with.
BONE-BED OF THE UPPER LUDLOW.
At the base of the Downton sandstones there occurs a bone-bed which deserves
especial notice as affording the most ancient example of fossil fish occurring
in any considerable quantity. It usually consists of one or two thin layers of
brown bony fragments near the junction of the Old Red Sandstone and the Ludlow
rocks, and was first observed by Sir R. Murchison near the town of Ludlow, where
it is three or four inches thick. It has since been traced to a distance of 45
miles from that point into Gloucestershire and other counties, and is commonly
not more than an inch thick, but varies to nearly a foot. Near Ludlow two bone-
beds are observable, with 14 feet of intervening strata full of Upper Ludlow
fossils. (Murchison's Siluria page 140.) At that point immediately above the
upper fish-bed numerous small globular bodies have been found, which were
determined by Dr. Hooker to be the sporangia of a cryptogamic land-plant,
probably lycopodiaceous.
(FIGURE 524. Onchus tenuistriatus, Agassiz. Bone-bed. Upper Silurian. Ludlow.)
(FIGURE 525. Shagreen-scales of a placoid fish, Thelodus parvidens, Agassiz.
Bone-bed, Upper Ludlow.)
(FIGURE 526. Plectrodus mirabilis, Agassiz. Bone-bed, Upper Ludlow.)
Most of the fish have been referred by Agassiz to his placoid order, some of
them to the genus Onchus, to which the spine (Figure 524) and the minute scales
(Figure 525) are supposed to belong. It has been suggested, however, that Onchus
may be one of those Acanthodian fish referred by Agassiz to his Ganoid order,
which are so characteristic of the base of the Old Red Sandstone in Forfarshire,
although the species of the Old Red are all different from these of the Silurian
beds now under consideration. The jaw and teeth of another predaceous genus
(Figure 526) have also been detected, together with some specimens of Pteraspis
Ludensis. As usual in bone-beds, the teeth and bones are, for the most part,
fragmentary and rolled.
GREY SANDSTONE AND MUDSTONE, ETC.
(FIGURE 527. Orthis elegantula, Dalm. Var. Orbicularis, Sowerby. Upper Ludlow.)
(FIGURE 528. Rhynchonella navicula, Sowerby. Ludlow Beds.)
The next subdivision of the Upper Ludlow consists of grey calcareous sandstone,
or very commonly a micaceous stone, decomposing into soft mud, and contains,
besides the shells mentioned above, Lingula cornea, Orthis orbicularis, a round
variety of O. elegantula, Modiolopsis platyphylla, Grammysia cingulata, all
characteristic of the Upper Ludlow. The lowest or mud-stone beds contain
Rhynchonella navicula (Figure 528), which is common to this bed and the Lower
Ludlow. As usual in Palaeozoic strata older than the coal, the brachiopodous
mollusca greatly outnumber the lamellibranchiate (see below); but the latter are
by no means unrepresented. Among other genera, for example, we observe Avicula
and Pterinea, Cardiola, Ctenodonta (sub-genus of Nucula), Orthonota,
Modiolopsis, and Palaearca.
Some of the Upper Ludlow sandstones are ripple-marked, thus affording evidence
of gradual deposition; and the same may be said of the accompanying fine
argillaceous shales, which are of great thickness, and have been provincially
named "mud-stones." In some of these shales stems of crinoidea are found in an
erect position, having evidently become fossil on the spots where they grew at
the bottom of the sea. The facility with which these rocks, when exposed to the
weather, are resolved into mud, proves that, notwithstanding their antiquity,
they are nearly in the state in which they were first thrown down.
b. LOWER LUDLOW BEDS.
(FIGURE 529. Pentamerus Knightii, Sowerby. Aymestry. One-half natural size.
a. View of both valves united.
b. Longitudinal section through both valves, showing the central plates or
septa.)
The chief mass of this formation consists of a dark grey argillaceous shale with
calcareous concretions, having a maximum thickness of 1000 feet. In some places,
and especially at Aymestry, in Herefordshire, a subcrystalline and argillaceous
limestone, sometimes 50 feet thick, overlies the shale. Sir R. Murchison
therefore classes this Aymestry limestone as holding an intermediate position
between the Upper and Lower Ludlow, but Mr. Lightbody remarks that at Mocktrie,
near Leintwardine, the Lower Ludlow shales, with their characteristic fossils,
occur both above and below a similar limestone. This limestone around Aymestry
and Sedgeley is distinguished by the abundance of Pentamerus Knightii, Sowerby
(Figure 529), also found in the Lower Ludlow and Wenlock shale. This genus of
brachiopoda was first found in Silurian strata, and is exclusively a palaeozoic
form. The name was derived from pente, five, and meros, a part, because both
valves are divided by a central septum, making four chambers, and in one valve
the septum itself contains a small chamber, making five. The size of these septa
is enormous compared with those of any other brachiopod shell; and they must
nearly have divided the animal into two equal halves; but they are,
nevertheless, of the same nature as the septa or plates which are found in the
interior of Spirifera, Terebratula, and many other shells of this order. Messrs.
Murchison and De Verneuil discovered this species dispersed in myriads through a
white limestone of Upper Silurian age, on the banks of the Is, on the eastern
flank of the Urals in Russia, and a similar species is frequent in Sweden.
(FIGURE 530. Lingula Lewisii, J. Sowb. Abberley Hills.)
(FIGURE 531. Rhynchonella (Terebratula) Wilsoni, Sowerby. Aymestry.)
(FIGURE 532. Atrypa reticularis, Linn. (Terebratula affinis, Min. Con.)
Aymestry.
a. Upper valve.
b. Lower valve.
c. Anterior margin of the valves.)
Three other abundant shells in the Aymestry limestone are, 1st, Lingula Lewisii
(Figure 530); second, Rhynchonella Wilsoni, Sowerby. (Figure 531), which is also
common to the Lower Ludlow and Wenlock limestone; third, Atrypa reticularis,
Linn. (Figure 532), which has a very wide range, being found in every part of
the Upper Silurian system, and even ranging up into the Middle Devonian series.
The Aymestry Limestone contains many shells, especially brachiopoda, corals,
trilobites, and other fossils, amounting on the whole to 74 species, all except
three or four being common to the beds either above or below.
(FIGURE 533. Phragmoceras ventricosum, J. Sowerby. (Orthoceras ventricosum,
Stein.) Aymestry; one-quarter natural size.)
(FIGURE 534. Lituites (Trochoceras) giganteus, J. Sowerby. Near Ludlow; also in
the Aymestry and Wenlock Limestones; 1/4 natural size.)
(FIGURE 535. Fragment of Orthoceras Ludense, J. Sowerby. Leintwardine,
Shropshire.)
The Lower Ludlow Shale contains, among other fossils, many large cephalopoda not
known in newer rocks, as the Phragmoceras of Broderip, and the Lituites of
Breynius (see Figures 533, 534). The latter is partly straight and partly
convoluted in a very flat spire. The Orthoceras Ludense (Figure 535), as well as
the cephalopod last mentioned, occurs in this member of the species.
A species of Graptolite, G. priodon, Bronn (Figure 545), occurs plentifully in
the Lower Ludlow. This fossil, referred, though somewhat doubtfully, to a form
of hydrozoid or sertularian polyp, has not yet been met with in strata above the
Silurian.
Star-fish, as Sir R. Murchison points out, are by no means rare in the Lower
Ludlow rock. These fossils, of which six extinct genera are now known in the
Ludlow series, represented by 18 species, remind us of various living forms now
found in our British seas, both of the families Asteriadae and Ophiuridae.
OLDEST KNOWN FOSSIL FISH.
Until 1859 there was no example of a fossil fish older than the bone-bed of the
Upper Ludlow, but in that year a specimen of Pteraspis was found at Church Hill,
near Leintwardine, in Shropshire, by Mr. J.E. Lee of Caerleon, F.G.S., in shale
below the Aymestry limestone, associated with fossil shells of the Lower Ludlow
formation-- shells which differ considerably from those characterising the Upper
Ludlow already described. This discovery is of no small interest as bearing on
the theory of progressive development, because, according to Professor Huxley,
the genus Pteraspis is allied to the sturgeon, and therefore by no means of low
grade in the piscine class.
It is a fact well worthy of notice that no remains of vertebrata have yet been
met with in any strata older than the Lower Ludlow.
When we reflect on the hundreds of Mollusks, Echinoderms, Trilobites, Corals,
and other fossils already obtained from more ancient Silurian formations, Upper,
Middle, and Lower, we may well ask whether any set of fossiliferous rocks newer
in the series were ever studied with equal diligence, and over so vast an area,
without yielding a single ichthyolite. Yet we must hesitate before we accept,
even on such evidence, so sweeping a conclusion, as that the globe, for ages
after it was inhabited by all the great classes of invertebrata, remained wholly
untenanted by vertebrate animals.
TABLE 26.2. DATES OF THE DISCOVERY OF DIFFERENT CLASSES OF FOSSIL VERTEBRATA;
SHOWING THE GRADUAL PROGRESS MADE IN TRACING THEM TO ROCKS OF HIGHER ANTIQUITY.
COLUMN 1: YEAR.
COLUMN 2: FORMATIONS.
COLUMN 3: GEOGRAPHICAL LOCALITIES.
MAMMALIA:
1798: Upper Eocene: Paris (Gypsum of Montmartre). (George Cuvier, Bulletin Soc.
Philom. 20.)
1818: Lower Oolite: Stonesfield. (In 1818, Cuvier, visiting the Museum of
Oxford, decided on the mammalian character of a jaw from Stonesfield. See also
above Chapter 19.)
1847: Upper Trias: Stuttgart. (Professor Plieninger. See above Chapter 21.)
AVES:
1782: Upper Eocene: Paris (Gypsum of Montmartre). (Cuvier, Ossemens Foss. Art.
"Oiseaux.")
1839: Lower Eocene: Isle of Sheppey (London Clay). (Professor Owen Geological
Transactions second series volume 6 page 203 1839.)
1854: Lower Eocene: Woolwich Beds. (Upper part of the Woolwich beds. Prestwich
Quarterly Geological Journal volume 10 page 157.)
1855: Lower Eocene: Meudon (Plastic Clay). (Gastornis Parisiensis. Owen
Quarterly Geological Journal volume 12 page 204 1856.)
1858: Chloritic Series, or Upper Greensand: Cambridge. (Coprolitic bed, in the
Upper Greensand. See above Chapter 17.)
1863: Upper Oolite: Solenhofen. (The Archaeopteryx macrura, Owen. See above
Chapter 19.)
REPTILIA:
1710: Permian (or Zechstein): Thuringia. (The fossil monitor of Thuringia
(Protosaurus Speneri, V. Meyer) was figured by Spener of Berlin in 1810.
(Miscel. Berlin.))
1844: Carboniferous: Saarbruck, near Treves. (See Chapter 23.)
PISCES:
1709: Permian (or Kupferschiefer): Thuringia. (Memorabilia Saxoniae Subterr.
Leipsic 1709.)
1793: Carboniferous (Mountain Limestone): Glasgow. (History of Rutherglen by
David Ure, 1793.)
1828: Devonian: Caithness. (Sedgwick and Murchison Geological Transactions
second series volume 3 page 141 1828.)
1840: Upper Ludlow: Ludlow. (Sir R. Murchison. See Chapter 26.)
1859: Lower Ludlow: Leintwardine. (See Chapter 26.)
Obs.-- The evidence derived from foot-prints, though often to be relied on, is
omitted in the above table, as being less exact than that founded on bones and
teeth.
In Table 26.2 a few dates are set before the reader of the discovery of
different classes of animals in ancient rocks, to enable him to perceive at a
glance how gradual has been our progress in tracing back the signs of vertebrata
to formations of high antiquity. Such facts may be useful in warning us not to
assume too hastily that the point which our retrospect may have reached at the
present moment can be regarded as fixing the date of the first introduction of
any one class of beings upon the earth.
2. WENLOCK FORMATION.
We next come to the Wenlock formation, which has been divided into Wenlock
limestone, Wenlock shale, and Woolhope limestone and Denbighshire grits.
a. WENLOCK LIMESTONE.
This limestone, otherwise well known to collectors by the name of the Dudley
Limestone, forms a continuous ridge in Shropshire, ranging for about 20 miles
from S.W. to N.E., about a mile distant from the nearly parallel escarpment of
the Aymestry limestone. This ridgy prominence is due to the solidity of the
rock, and to the softness of the shales above and below it. Near Wenlock it
consists of thick masses of grey subcrystalline limestone, replete with corals,
encrinites, and trilobites. It is essentially of a concretionary nature; and the
concretions, termed "ball-stones" in Shropshire, are often enormous, even 80
feet in diameter. They are of pure carbonate of lime, the surrounding rock being
more or less argillaceous (Murchison's Siluria chapter 6.) Sometimes in the
Malvern Hills this limestone, according to Professor Phillips, is oolitic.
(FIGURE 536. Halysites catenularius, Linn. sp. Upper and Lower Silurian.)
(FIGURE 537. Favosites Gothlandica, Lam. Dudley.
a. Portion of a large mass; less than the natural size.
b. Magnified portion, to show the pores and the partitions in the tubes.)
(FIGURE 538. Omphyma turbinatum, Linn. Sp. (Cyathophyllum, Goldfuss) Wenlock
Limestone, Shropshire.)
Among the corals, in which this formation is so rich, 53 species being known,
the "chain-coral," Halysites catenularius (Figure 536), may be pointed out as
one very easily recognised, and widely spread in Europe, ranging through all
parts of the Silurian group, from the Aymestry limestone to near the bottom of
the Llandeilo rocks. Another coral, the Favosites Gothlandica (Figure 537), is
also met with in profusion in large hemispherical masses, which break up into
columnar and prismatic fragments, like that here figured (Figure 537, b).
Another common form in the Wenlock limestone is the Omphyma turbinatum (Figure
538), which, like many of its modern companions, reminds us of some cup-corals;
but all the Silurian genera belong to the palaeozoic type before mentioned
(Chapter 24), exhibiting the quadripartite arrangement of the septalamellae
within the cup.
(FIGURE 539. Pseudocrintes bifasciatus, Pearce. Wenlock Limestone, Dudley.)
Among the numerous Crinoids, several peculiar species of Cyathocrinus (for genus
see Figures 478, 479) contribute their calcareous stems, arms, and cups towards
the composition of the Wenlock limestone. Of Cystideans there are a few very
remarkable forms, most of them peculiar to the Upper Silurian formation, as, for
example, the Pseudocrinites, which was furnished with pinnated fixed arms, as
represented in Figure 539. (E. Forbes Mem. Geological Survey volume 2 page 496.)
(FIGURE 540. Strophomena (Leptaena) depressa, Sowerby. Wenlock and Ludlow
Rocks.)
The Brachiopoda are, many of them, of the same species as those of the Aymestry
limestone; as, for example, Atrypa reticularis (Figure 532), and Strophomena
depressa (Figure 540); but the latter species ranges also from the Ludlow rocks,
through the Wenlock shale, to the Caradoc Sandstone.
(FIGURE 541. Calymene Blumenbachii, Brong. Ludlow, Wenlock, and Bala beds.)
(FIGURE 542. Phacops (Asaphus) caudatus, Brong. Wenlock and Ludlow Rocks.)
(FIGURE 543. Sphaerexochus mirus, Beyrich; coiled up. Wenlock Limestone, Dudley;
also found in Ohio, North America.)
(FIGURE 544. Homalonotus delphinocephalus, Konig. Wenlock Limestone, Dudley
Castle.)
The crustaceans are represented almost exclusively by Trilobites, which are very
conspicuous, 22 being peculiar. The Calymene Blumenbachii (Figure 541), called
the "Dudley Trilobite," was known to collectors long before its true place in
the animal kingdom was ascertained. It is often found coiled up like the common
Oniscus or wood-louse, and this is so usual a circumstance among certain genera
of trilobites as to lead us to conclude that they must have habitually resorted
to this mode of protecting themselves when alarmed. The other common species is
the Phacops caudatus (Asaphus caudatus), Brong. (see Figure 542), and this is
conspicuous for its large size and flattened form. Sphaerexochus mirus (Figure
543) is almost a globe when rolled up, the forehead or glabellum of this species
being extremely inflated. The Homalonotus, a form of Trilobite in which the
tripartite division of the dorsal crust is almost lost (see Figure 544), is very
characteristic of this division of the Silurian series.
WENLOCK SHALE.
(FIGURE 545. Graptolithus priodon, Bronn. Ludlow and Wenlock shales.)
The Wenlock Shale, observes Sir R. Murchison, is infinitely the largest and most
persistent member of the Wenlock formation, for the limestone often thins out
and disappears. The shale, like the Lower Ludlow, often contains elliptical
concretions of impure earthy limestone. In the Malvern district it is a mass of
finely levigated argillaceous matter, attaining, according to Professor
Phillips, a thickness of 640 feet, but it is sometimes more than 1000 feet thick
in Wales, and is worked for flag-stones and slates. The prevailing fossils,
besides corals and trilobites, and some crinoids, are several small species of
Orthis, Cardiola, and numerous thin-shelled species of Orthoceratites.
About six species of Graptolite, a peculiar group of sertularian fossils before
alluded to as being confined to Silurian rocks, occur in this shale. Of fossils
of this genus, which is very characteristic of the Lower Silurian, I shall again
speak in the sequel.
b. WOOLHOPE BEDS.
Though not always recognised as a separate subdivision of the Wenlock, the
Woolhope beds, which underlie the Wenlock shale, are of great importance.
Usually they occur as massive or nodular limestones, underlaid by a fine shale
or flag-stone; and in other cases, as in the noted Denbighshire sandstones, as a
coarse grit of very great thickness. This grit forms mountain ranges through
North and South Wales, and is generally marked by the great sterility of the
soil where it occurs. It contains the usual Wenlock fossils, but with the
addition of some common in the uppermost Ludlow rock, such as Chonetes lata and
Bellerophon trilobatus. The chief fossils of the Woolhope limestone are Illaenus
Barriensis, Homalonotus delphinocephalus (Figure 544), Strophomena imbrex, and
Rhynchonella Wilsoni (Figure 531). The latter attains in the Woolhope beds an
unusual size for the species, the specimens being sometimes twice as large as
those found in the Wenlock limestone.
In some places below the Wenlock formation there are shales of a pale or purple
colour, which near Tarannon attain a thickness of about 1000 feet; they can be
traced through Radnor and Montgomery to North Wales, according to Messrs. Jukes
and Aveline. By the latter geologist they have been identified with certain
shales above the May-Hill Sandstone, near Llandovery, but, owing to the extreme
scarcity of fossils, their exact position remains doubtful.
3. LLANDOVERY GROUP-- BEDS OF PASSAGE.
We now come to beds respecting the classification of which there has been much
difference of opinion, and which in fact must be considered as beds of passage
between Upper and Lower Silurian. I formerly adopted the plan of those who class
them as Middle Silurian, but they are scarcely entitled to this distinction,
since after about 1400 Silurian species have been compared the number peculiar
to the group in question only gives them an importance equal to such minor
subdivisions as the Ludlow or Bala groups. I therefore prefer to regard them as
the base of the Upper Silurian, to which group they are linked by more than
twice as many species as to the Lower Silurian. By this arrangement the line of
demarkation between the two great divisions, though confessedly arbitrary, is
less so than by any other. They are called Llandovery Rocks, from a town in
South Wales, in the neighbourhood of which they are well developed, and where,
especially at a hill called Noeth Grug, in spite of several faults, their
relations to one another can be clearly seen.
a. UPPER LLANDOVERY OR MAY-HILL SANDSTONE.
(FIGURE 546. Pentamerus oblongus, Sowerby. Upper and Lower Llandovery beds.
a, b. Views of the shell itself, from figures in Murchison's Sil. Syst.
c. Cast with portion of shell remaining, and with the hollow of the central
septum filled with spar.
d. Internal cast of a valve, the space once occupied by the septum being
represented by a hollow in which is seen a cast of the chamber within the
septum.)
(FIGURE 547. Stricklandinia (Pentamerus) lirata, Sowerby.)
The May-Hill group, which has also been named "Upper Llandovery," by Sir R.
Murchison, ranges from the west of the Longmynd to Builth, Llandovery, and
Llandeilo, and to the sea in Marlow's Bay, where it is seen in the cliffs. It
consists of brownish and yellow sandstones with calcareous nodules, having
sometimes a conglomerate at the base derived from the waste of the Lower
Silurian rocks. These May-Hill beds were formerly supposed to be part of the
Caradoc formation, but their true position was determined by Professor Sedgwick
to be at the base of the Upper Silurian proper. (Quarterly Geological Journal
volume 4 page 215 1853.) The more calcareous portions of the rock have been
called the Pentamerus limestone, because Pentamerus oblongus (Figure 546) is
very abundant in them. It is usually accompanied by P. (Stricklandinia) lirata
(Figure 547); both forms have a wide geographical range, being also met with in
the same part of the Silurian series in Russia and the United States.
About 228 species of fossils are known in the May-Hill division, more than half
of which are Wenlock species. They consist of trilobites of the genera Illaenus
and Calymene; Brachiopods of the genera Orthis, Atrypa, Leptaena, Pentamerus,
Strophomena, and others; Gasteropods of the genera Turbo, Murchisonia (for
genus, see Figure 567), and Bellerophon; and Pteropods of the genus Conularia.
The Brachiopods, of which there are 66 species, are almost all Upper Silurian.
(FIGURE 548. Tentaculites annulatus, Schlot. Interior casts in sandstone. Upper
Llandovery, Eastnor Park, near Malvern. Natural size and magnified.)
Among the fossils of the May-Hill shelly sandstone at Malvern, Tentaculites
annulatus (Figure 548), an annelid, probably allied to Serpula, is found.
LOWER LLANDOVERY ROCKS.
Below the May-Hill Group are the Lower Llandovery Rocks, which consist chiefly
of hard slaty rocks, and beds of conglomerate from 600 to 1000 feet in
thickness. The fossils, which are somewhat rare in the lower beds, consist of
128 known species, only eleven of which are peculiar, 83 being common to the
May-Hill group above, and 93 common to the rocks below. Stricklandinia
(Pentamerus) levis, which is common in the Lower Llandovery, becomes rare in the
Upper, while Pentamerus oblongus (Figure 546), which is the characteristic shell
of the Upper Llandovery, occurs but seldom in the Lower.
LOWER SILURIAN ROCKS.
The Lower Silurian has been divided into, first, the Bala Group; secondly, the
Llandeilo flags; and, thirdly, the Arenig or Lower Llandeilo formation.
BALA AND CARADOC BEDS.
(FIGURE 549. Orthis tricenaria, Conrad. New York; Canada. 1/2 natural size.)
(FIGURE 550. Orthis vespertilio, Sowerby. Shropshire, N. and S. Wales. One-half
natural size.)
(FIGURE 551. Orthis (Strophomena) grandis, Sowerby. Two-thirds natural size.
Caradoc Beds, Horderley, Shropshire, and Coniston, Lancashire.)
The Caradoc sandstone was originally so named by Sir R.I. Murchison from the
mountain called Caer Caradoc, in Shropshire; it consists of shelly sandstones of
great thickness, and sometimes containing much calcareous matter. The rock is
frequently laden with the beautiful trilobite called by Murchison Trinucleus
Caractaci (see Figure 553), which ranges from the base to the summit of the
formation, usually accompanied by Strophomena grandis (see Figure 551), and
Orthis vespertilio (Figure 550), with many other fossils.
BRACHIOPODA.
Nothing is more remarkable in these beds, and in the Silurian strata generally
of all countries, than the preponderance of brachiopoda over other forms of
mollusca. Their proportional numbers can by no means be explained by supposing
them to have inhabited seas of great depth, for the contrast between the
palaeozoic and the present state of things has not been essentially altered by
the late discoveries made in our deep-sea dredgings. We find the living
brachiopoda so rare as to form about one forty-fourth of the whole bivalve
fauna, whereas in the Lower Silurian rocks of which we are now about to treat,
and where the brachiopoda reach their maximum, they are represented by more than
twice as many species as the Lamellibranchiate bivalves.
There may, indeed, be said to be a continued decrease of the proportional number
of this lower tribe of mollusca as we proceed from older to newer rocks. In the
British Devonian, for example, the Brachiopoda number 99, the Lamellibranchiata
58; while in the Carboniferous their proportions are more than reversed, the
Lamellibranchiata numbering 334 species, and the Brachiopoda only 157. In the
Secondary or Cainozoic formations the preponderance of the higher grade of
bivalves becomes more and more marked, till in the tertiary strata it approaches
that observed in the living creation.
While on this subject, it may be useful to the student to know that a Brachiopod
differs from ordinary bivalves, mussels, cockles, etc., in being always equal-
sided and never quite equi-valved; the form of each valve being symmetrical, it
may be divided into two equal parts by a line drawn from the apex to the centre
of the margin.
TRILOBITES.
In the Bala and Caradoc beds the trilobites reach their maximum, being
represented by 111 species referred to 23 genera.
(FIGURE 552. Young individuals of Trinucleus concentricus (T. ornatus, Barr.).
a. Youngest state. Natural size and magnified; the body rings not at all
developed.
b. A little older. One thorax joint.
c. Still more advanced. Three thorax joints. The fourth, fifth, and sixth
segments are successively produced, probably each time the animal moulted its
crust.)
(FIGURE 553. Trinucleus concentricus, Eaton. Syn. T. Caractaci, Murch. Ireland;
Wales; Shropshire; North America; Bohemia.)
Burmeister, in his work on the organisation of trilobites, supposes that they
swam at the surface of the water in the open sea and near coasts, feeding on
smaller marine animals, and to have had the power of rolling themselves into a
ball as a defence against injury. He was also of opinion that they underwent
various transformations analogous to those of living crustaceans. M. Barrande,
author of an admirable work on the Silurian rocks of Bohemia, confirms the
doctrine of their metamorphosis, having traced more than twenty species through
different stages of growth from the young state just after its escape from the
egg to the adult form. He has followed some of them from a point in which they
show no eyes, no joints, or body rings, and no distinct tail, up to the complete
form with the full number of segments. This change is brought about before the
animal has attained a tenth part of its full dimensions, and hence such minute
and delicate specimens are rarely met with. Some of his figures of the
metamorphoses of the common Trinucleus are copied in Figures 552 and 553. It was
not till 1870 that Mr. Billings was enabled, by means of a specimen found in
Canada, to prove that the trilobite was provided with eight legs.
(FIGURE 554. Palaeaster asperimus, Salt. Caradoc, Welshpool.)
(FIGURE 555. Echinosphaeronites balticus, Eichwald. (Of the family Cystideae.)
a. Mouth.
b. Point of attachment of stem. Lower Silurian S. and N. Wales.)
It has been ascertained that a great thickness of slaty and crystalline rocks of
South Wales, as well as those of Snowdon and Bala, in North Wales, which were
first supposed to be of older date than the Silurian sandstones and mudstones of
Shropshire, are in fact identical in age, and contain the same organic remains.
At Bala, in Merionethshire, a limestone rich in fossils occurs, in which two
genera of star-fish, Protaster and Palaeaster, are found; the fossil specimen of
the latter (Figure 554) being almost as uncompressed as if found just washed up
on the sea-beach. Besides the star-fish there occur abundance of those peculiar
bodies called Cystideae. They are the Sphaeronites of old authors, and were
considered by Professor E. Forbes as intermediate between the crinoids and
echinoderms. The Echinosphaeronite here represented (Figure 555) is
characteristic of the Caradoc beds in Wales, and of their equivalents in Sweden
and Russia.
With it have been found several other genera of the same family, such as
Sphaeronites, Hemicosmites, etc. Among the mollusca are Pteropods of the genus
Conularia of large size (for genus, see Figure 518). About eleven species of
Graptolite are reckoned as belonging to this formation; they are chiefly found
in peculiar localities where black mud abounded. The formation, when traced into
South Wales and Ireland, assumes a greatly altered mineral aspect, but still
retains its characteristic fossils. The known fauna of the Bala group comprises
565 species, 352 of which are peculiar, and 93, as before stated, are common to
the overlying Llandovery rocks. It is worthy of remark that, when it occurs
under the form of trappean tuff (volcanic ashes of De la Beche), as in the crest
of Snowdon, the peculiar species which distinguish it from the Llandeilo beds
are still observable. The formation generally appears to be of shallow-water
origin, and in that respect is contrasted with the group next to be described.
Professor Ramsay estimates the thickness of the Bala Beds, including the
contemporaneous volcanic rocks, stratified and unstratified, as being from
10,000 to 12,000 feet.
LLANDEILO FLAGS.
(FIGURE 556. Didymograpsus (Graptolites) Murchisonii, Beck. Llandeilo flags,
Wales.)
The Lower Silurian strata were originally divided by Sir R. Murchison into the
upper group already described, under the name of Caradoc Sandstone, and a lower
one, called, from a town in Carmarthenshire, the Llandeilo flags. The last
mentioned strata consist of dark-coloured micaceous flags, frequently
calcareous, with a great thickness of shales, generally black, below them. The
same beds are also seen at Builth, in Radnorshire, where they are
interstratified with volcanic matter.
(FIGURE 557. Diplograpsus pristis, Hisinger. Llandeilo beds, Waterford.)
(FIGURE 558. Rastrites peregrinus, Barrande. Scotland; Bohemia; Saxony.
Llandeilo flags.)
(FIGURE 559. Diplograpsus folium, Hisinger. Dumfriesshire; Sweden. Llandeilo
flags.)
A still lower part of the Llandeilo rocks consists of a black carbonaceous slate
of great thickness, frequently containing sulphate of alumina, and sometimes, as
in Dumfriesshire, beds of anthracite. It has been conjectured that this
carbonaceous matter may be due in great measure to large quantities of imbedded
animal remains, for the number of Graptolites included in these slates was
certainly very great. In Great Britain eleven genera and about 40 species of
Graptolites occur in the Llandeilo flags and underlying Arenig beds. The double
Graptolites, or those with two rows of cells, such as Diplograpsus (Figure 557),
are conspicuous.
The brachiopoda of the Llandeilo flags, which number 47 species, are in the main
the same as those of the Caradoc Sandstone, but the other mollusca are in great
part of different species.
(FIGURE 560. Orthoceras duplex, Wahlenberg. Russia and Sweden. (From Murchison's
Siluria.))
(FIGURE 561. Asaphus tyrannus, Murchison. Llandeilo; Bishop's Castle; etc.)
(FIGURE 562. Ogygia Buchii, Burm. Syn. Asaphus Buchii, Brongn. Builth,
Radnorshire; Llandeilo, Carmarthenshire.)
In Europe generally, as, for example, in Sweden and Russia, no shells are so
characteristic of this formation as Orthoceratites, usually of great size, and
with a wide siphuncle placed on one side instead of being central (see Figure
560). Among other Cephalopods in the Llandeilo flags is Cyrtoceras; in the same
beds also are found Bellerophon (see Figure 488) and some Pteropod shells
(Conularia, Theca, etc.), also in spots where sand abounded, lamellibranchiate
bivalves of large size. The Crustaceans were plentifully represented by the
Trilobites, which appear to have swarmed in the Silurian seas just as crabs and
shrimps do in our own; no less than 263 species have been found in the British
Silurian fauna. The genera Asaphus (Figure 561), Ogygia (Figure 562), and
Trinucleus (Figures 552 and 553) form a marked feature of the rich and varied
Trilobitic fauna of this age.
Beneath the black slates above described of the Llandeilo formation, Graptolites
are still found in great variety and abundance, and the characteristic genera of
shells and trilobites of the Lower Silurian rocks are still traceable downward,
in Shropshire, Cumberland, and North and South Wales, through a vast depth of
shaly beds, in some districts interstratified with trappean formations of
contemporaneous origin; these consist of tuffs and lavas, the tuffs being formed
of such materials as are ejected from craters and deposited immediately on the
bed of the ocean, or washed into it from the land. According to Professor
Ramsay, their thickness is about 3300 feet in North Wales, including those of
the Lower Llandeilo. The lavas are feldspathic, and of porphyritic structure,
and, according to the same authority, of an aggregate thickness of 2500 feet.
ARENIG OR STIPER-STONES GROUP (LOWER LLANDEILO OF MURCHISON).
(FIGURE 563. Arenicolites linearis, Hall. Arenig beds, Stiper-Stones.
a. Parting between the beds, or planes of bedding.)
(FIGURE 564. Didymograpsus geminus, Hisinger, sp. Sweden.)
Next in the descending order are the shales and sandstones in which the
quartzose rocks called Stiper-Stones in Shropshire occur. Originally these
Stiper-Stones were only known as arenaceous quartzose strata in which no organic
remains were conspicuous, except the tubular burrows of annelids (see Figure
563, Arenicolites linearis), which are remarkably common in the Lowest Silurian
in Shropshire, and in the State of New York, in America. They have already been
alluded to as occurring by thousands in the Silurian strata unconformably
overlying the Cambrian, in the mountain of Queenaig, in Sutherlandshire (Figure
82). I have seen similar burrows now made on the retiring of the tides in the
sands of the Bristol Channel, near Minehead, by lob-worms which are dug out by
fishermen and used as bait. When the term Silurian was given by Sir R.
Murchison, in 1835, to the whole series, he considered the Stiper-Stones as the
base of the Silurian system, but no fossil fauna had then been obtained, such as
could alone enable the geologist to draw a line between this member of the
series and the Llandeilo flags above, or a vast thickness of rock below, which
was seen to form the Longmynd hills, and was called "unfossiliferous graywacke."
Professor Sedgwick had described, in 1843, strata now ascertained to be of the
same age as largely developed in the Arenig mountain, in Merionethshire; and the
Skiddaw slates in the Lake-District of Cumberland, studied by the same author,
were of corresponding date, though the number of fossils was, in both cases, too
few for the determination of their true chronological relations. The subsequent
researches of Messrs. Sedgwick and Harkness, in Cumberland, and of Sir R.I.
Murchison and the Government surveyors in Shropshire, have increased the species
to more than sixty. These were examined by Mr. Salter, and shown in the third
edition of "Siluria" (page 52, 1859) to be quite distinct from the fossils of
the overlying Llandeilo flags. Among these the Obolella plumbea, Aeglina
binodosa, Ogygia Selwynii, and Didymograpsus geminus (Figure 564), and D.
Hirundo, are characteristic.
But, although the species are distinct, the genera are the same as those which
characterise the Silurian rocks above, and none of the characteristic primordial
or Cambrian forms, presently to be mentioned, are intermixed. The same may be
said of a set of beds underlying the Arenig rocks at Ramsay Island and other
places in the neighbourhood of St. David's. These beds, which have only lately
become known to us through the labours of Dr. Hicks (Transactions of the British
Association 1866. Proceedings of the Liverpool Geological Society 1869.),
present already twenty new species, the greater part of them allied generically
to the Arenig rocks. This Arenig group may therefore be conveniently regarded as
the base of the great Silurian system, a system which, by the thickness of its
strata and the changes in animal life of which it contains the record, is more
than equal in value to the Devonian, or Carboniferous, or other principal
divisions, whether of primary or secondary date.
It would be unsafe to rely on the mere thickness of the strata, considered apart
from the great fluctuations in organic life which took place between the era of
the Llandeilo and that of the Ludlow formation, especially as the enormous pile
of Silurian rocks observed in Great Britain (in Wales more particularly) is
derived in great part from igneous action, and is not confined to the ordinary
deposition of sediment from rivers or the waste of cliffs.
In volcanic archipelagoes, such as the Canaries, we see the most active of all
known causes, aqueous and igneous, simultaneously at work to produce great
results in a comparatively moderate lapse of time. The outpouring of repeated
streams of lava-- the showering down upon land and sea of volcanic ashes-- the
sweeping seaward of loose sand and cinders, or of rocks ground down to pebbles
and sand, by rivers and torrents descending steeply inclined channels-- the
undermining and eating away of long lines of sea-cliff exposed to the swell of a
deep and open ocean-- these operations combine to produce a considerable volume
of superimposed matter, without there being time for any extensive change of
species. Nevertheless, there would seem to be a limit to the thickness of stony
masses formed even under such favourable circumstances, for the analogy of
tertiary volcanic regions lends no countenance to the notion that sedimentary
and igneous rocks 25,000, much less 45,000 feet thick, like those of Wales,
could originate while one and the same fauna should continue to people the
earth. If, then, we allow that about 25,000 feet of matter may be ascribed to
one system, such as the Silurian, as above described, we may be prepared to
discover in the next series of subjacent rocks a distinct assemblage of species,
or even in great part of genera, of organic remains. Such appears to be the
fact, and I shall therefore conclude with the Arenig beds my enumeration of the
Silurian formations in Great Britain, and proceed to say something of their
foreign equivalents, before treating of rocks older than the Silurian.
SILURIAN STRATA OF THE CONTINENT OF EUROPE.
When we turn to the continent of Europe, we discover the same ancient series
occupying a wide area, but in no region as yet has it been observed to attain
great thickness. Thus, in Norway and Sweden, the total thickness of strata of
Silurian age is considerably less than 1000 feet, although the representatives
both of the Upper and Lower Silurian of England are not wanting there. In Russia
the Silurian strata, so far as they are yet known, seem to be even of smaller
vertical dimensions than in Scandinavia, and they appear to consist chiefly of
the Llandovery group, or of a limestone containing Pentamerus oblongus, below
which are strata with fossils corresponding to those of the Llandeilo beds of
England. The lowest rock with organic remains yet discovered is "the Ungulite or
Obolus grit" of St. Petersburg, probably coeval with the Llandeilo flags of
Wales.
(Figures 565 and 566. Shells of the lowest known Fossiliferous Beds in Russia.
(FIGURE 565. Siphonotreta unguiculata, Eichwald. From the Lowest Silurian
Sandstone, "Obolus grits," of St. Petersburg.
a. Outside of perforated valve.
b. Interior of same, showing the termination of the foramen within. (Davidson.))
(FIGURE 566. Obolus Apollinis, Eichwald. From the same locality.
a. Interior of the larger or ventral valve.
b. Exterior of the upper (dorsal) valve. (Davidson, "Palaeontographic
Monograph.")))
The shales and grits near St. Petersburg, above alluded to, contain green grains
in their sandy layers, and are in a singularly unaltered state, taking into
account their high antiquity. The prevailing Brachiopods consist of the Obolus
or Ungulite of Pander, and a Siphonotreta (Figures 565, 566). Notwithstanding
the antiquity of this Russian formation, it should be stated that both of these
genera of brachiopods have been also found in the Upper Silurian of England,
i.e. In the Wenlock limestone.
Among the green grains of the sandy strata above-mentioned, Professor Ehrenberg
announced in 1854 his discovery of remains of foraminifera. These are casts of
the cells; and among five or six forms three are considered by him as referable
to existing genera (e.g., Textularia, Rotalia, and Guttulina).
SILURIAN STRATA OF THE UNITED STATES.
Table 26.3. SUBDIVISIONS OF THE SILURIAN STRATA OF NEW YORK. (Strata below the
Oriskany sandstone or base of the Devonian.)
COLUMN 1: NEW YORK NAMES.
COLUMN 2: BRITISH EQUIVALENTS.
1. Upper Pentamerus Limestone: Upper Silurian (or Ludlow and Wenlock
formations).
2. Encrinal Limestone: Upper Silurian (or Ludlow and Wenlock formations).
3. Delthyris Shaly Limestone: Upper Silurian (or Ludlow and Wenlock formations).
4. Pentamerus and Tentaculite Limestones: Upper Silurian (or Ludlow and Wenlock
formations).
5. Water Lime Group: Upper Silurian (or Ludlow and Wenlock formations).
6. Onondaga Salt Group: Upper Silurian (or Ludlow and Wenlock formations).
7. Niagara Group: Upper Silurian (or Ludlow and Wenlock formations).
8. Clinton Group: Beds of Passage, Llandovery Group.
9. Medina Sandstone: Beds of Passage, Llandovery Group.
10. Oneida Conglomerate: Beds of Passage, Llandovery Group.
11. Gray Sandstone: Beds of Passage, Llandovery Group.
12. Hudson River Group: Lower Silurian (or Caradoc and Bala, Llandeilo and
Arenig Formations).
13. Trenton Limestone: Lower Silurian (or Caradoc and Bala, Llandeilo and Arenig
Formations).
14. Black-River Limestone: Lower Silurian (or Caradoc and Bala, Llandeilo and
Arenig Formations).
15. Bird's-eye Limestone: Lower Silurian (or Caradoc and Bala, Llandeilo and
Arenig Formations).
16. Chazy Limestone: Lower Silurian (or Caradoc and Bala, Llandeilo and Arenig
Formations).
17. Calciferous Sandstone: Lower Silurian (or Caradoc and Bala, Llandeilo and
Arenig Formations).
The Silurian formations can be advantageously studied in the States of New York,
Ohio, and other regions north and south of the great Canadian lakes. Here they
are often found, as in Russia, nearly in horizontal position, and are more rich
in well-preserved fossils than in almost any spot in Europe. In the State of New
York, where the succession of the beds and their fossils have been most
carefully worked out by the Government surveyors, the subdivisions given in the
first column of Table 26.3 have been adopted.
In the second column of the same table I have added the supposed British
equivalents. All Palaeontologists, European and American, such as MM. De
Verneuil, D. Sharpe, Professor Hall, E. Billings, and others, who have entered
upon this comparison, admit that there is a marked general correspondence in the
succession of fossil forms, and even species, as we trace the organic remains
downward from the highest to the lowest beds; but it is impossible to parallel
each minor subdivision.
That the Niagara Limestone, over which the river of that name is precipitated at
the great cataract, together with its underlying shales, corresponds to the
Wenlock limestone and shale of England there can be no doubt. Among the species
common to this formation in America and Europe are Calymene Blumenbachii,
Homalonotus delphinocephalus (Figure 544), with several other trilobites;
Rhynchonella Wilsoni, Figure 531, and Retzia cuneata; Orthis elegantula,
Pentamerus galeatus, with many more brachiopods; Orthoceras annulatum, among the
cephalopodous shells; and Favosites gothlandica, with other large corals.
The Clinton Group, containing Pentamerus oblongus and Stricklandinia, and
related more nearly by its fossil species with the beds above than with those
below, is the equivalent of the Llandovery Group or beds of passage.
(FIGURE 567. Murchisonia gracilis, Hall. A fossil characteristic of the Trenton
Limestone. The genus is common in Lower Silurian rocks.)
The Hudson River Group, and the Trenton Limestone, agree palaeontologically with
the Caradoc or Bala group, containing in common with them several species of
trilobites, such as Asaphus (Isotelus) gigas, Trinucleus concentricus (Figure
553); and various shells, such as Orthis striatula, Orthis biforata (or O.
lynx), O. porcata (O. occidentalis of Hall), and Bellerophon bilobatus. In the
Trenton limestone occurs Murchisonia gracilis, Figure 567, a fossil also common
to the Llandeilo beds in England.
Mr. D. Sharpe, in his report on the mollusca collected by me from these strata
in North America (Quarterly Geological Journal volume 4.), has concluded that
the number of species common to the Silurian rocks on both sides of the Atlantic
is between 30 and 40 per cent; a result which, although no doubt liable to
future modification, when a larger comparison shall have been made, proves,
nevertheless, that many of the species had a wide geographical range. It seems
that comparatively few of the gasteropods and lamellibranchiate bivalves of
North America can be identified specifically with European fossils, while no
less than two-fifths of the brachiopoda, of which my collection chiefly
consisted, are the same. In explanation of these facts, it is suggested that
most of the recent brachiopoda (especially the orthidiform ones) are inhabitants
of deep water, and that they may have had a wider geographical range than shells
living near shore. The predominance of bivalve mollusca of this peculiar class
has caused the Silurian period to be sometimes styled "the age of brachiopods."
In Canada, as in the State of New York, the Potsdam Sandstone underlies the
above-mentioned calcareous rocks, but contains a different suite of fossils, as
will be hereafter explained. In parts of the globe still more remote from Europe
the Silurian strata have also been recognised, as in South America, Australia,
and India. In all these regions the facies of the fauna, or the types of organic
life, enable us to recognise the contemporaneous origin of the rocks; but the
fossil species are distinct, showing that the old notion of a universal
diffusion throughout the "primaeval seas" of one uniform specific fauna was
quite unfounded, geographical provinces having evidently existed in the oldest
as in the most modern times.
CHAPTER XXVII.
CAMBRIAN AND LAURENTIAN GROUPS.
Classification of the Cambrian Group, and its Equivalent in Bohemia.
Upper Cambrian Rocks.
Tremadoc Slates and their Fossils.
Lingula Flags.
Lower Cambrian Rocks.
Menevian Beds.
Longmynd Group.
Harlech Grits with large Trilobites.
Llanberis Slates.
Cambrian Rocks of Bohemia.
Primordial Zone of Barrande.
Metamorphosis of Trilobites.
Cambrian Rocks of Sweden and Norway.
Cambrian Rocks of the United States and Canada.
Potsdam Sandstone.
Huronian Series.
Laurentian Group, upper and lower.
Eozoon Canadense, oldest known Fossil.
Fundamental Gneiss of Scotland.
CAMBRIAN GROUP.
The characters of the Upper and Lower Silurian rocks were established so fully,
both on stratigraphical and palaeontological data, by Sir Roderick Murchison
after five years' labour, in 1839, when his "Silurian System" was published,
that these formations could from that period be recognised and identified in all
other parts of Europe and in North America, even in countries where most of the
fossils differed specifically from those of the classical region in Britain,
where they were first studied.
TABLE 27.1. SHOWING THE SUCCESSION OF THE STRATA IN ENGLAND AND WALES WHICH
BELONG TO THE CAMBRIAN GROUP OR THE FOSSILIFEROUS ROCKS OLDER THAN THE ARENIG OR
LOWER LLANDEILO ROCKS:
UPPER CAMBRIAN.
TREMADOC SLATES. (Primordial of Barrande in part.)
LINGULA FLAGS. (Primordial of Barrande.)
LOWER CAMBRIAN.
MENEVIAN BEDS. (Primordial of Barrande.)
LONGMYND GROUP.
a. Harlech Grits.
b. Llanberis slates.
While Sir R.I. Murchison was exploring in 1833, in Shropshire and the borders of
Wales, the strata which in 1835 he first called Silurian, Professor Sedgwick was
surveying the rocks of North Wales, which both these geologists considered at
that period as of older date, and for which in 1836 Sedgwick proposed the name
of Cambrian. It was afterwards found that a large portion of the slaty rocks of
North Wales, which had been considered as more ancient than the Llandeilo beds
and Stiper-Stones before alluded to, were, in reality, not inferior in position
to those Lower Silurian beds of Murchison, but merely extensive undulations of
the same, bearing fossils identical in species, though these were generally
rarer and less perfectly preserved, owing to the changes which the rocks had
undergone from metamorphic action. To such rocks the term "Cambrian" was no
longer applicable, although it continued to be appropriate to strata inferior to
the Stiper-Stones, and which were older than those of the Lower Silurian group
as originally defined. It was not till 1846 that fossils were found in Wales in
the Lingula flags, the place of which will be seen in Table 27.1. By this time
Barrande had already published an account of a rich collection of fossils which
he had discovered in Bohemia, portions of which he recognised as of
corresponding age with Murchison's Upper and Lower Silurian, while others were
more ancient, to which he gave the name of "Primordial," for the fossils were
sufficiently distinct to entitle the rocks to be referred to a new period. They
consisted chiefly of trilobites of genera distinct from those occurring in the
overlying Silurian formations. These peculiar genera were afterwards found in
rocks holding a corresponding position in Wales, and I shall retain for them the
term Cambrian, as recent discoveries in our own country seem to carry the first
fauna of Barrande, or his primordial type, even into older strata than any which
he found to be fossiliferous in Bohemia.
The term primordial was intended to express M. Barrande's own belief that the
fossils of the rocks so-called afforded evidence of the first appearance of
vital phenomena on this planet, and that consequently no fossiliferous strata of
older date would or could ever be discovered. The acceptance of such a
nomenclature would seem to imply that we despaired of extending our discoveries
of new and more ancient fossil groups at some future day when vast portions of
the globe, hitherto unexplored, should have been thoroughly surveyed. Already
the discovery of the Laurentian Eozoon in Canada, presently to be mentioned,
discountenances such views.
UPPER CAMBRIAN.
TREMADOC SLATES.
(FIGURE 568. Theca (Cleidotheca) operculata. Lower Tremadoc beds. Tremadoc.)
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