abundant in tropical climates. They usually creep on the ground, but
some stand erect, as the _Lycopodium densum_ from New Zealand (see Fig.
456), which attains a height of three feet.
In the Carboniferous strata of Coalbrook Dale, and in many other
coal-fields, elongated cylindrical bodies, called fossil cones, named
_Lepidostrobus_ by M. Adolphe Brongniart, are met with. (See Fig. 457.)
They often form the nucleus of concretionary balls of clay-ironstone,
and are well preserved, exhibiting a conical axis, around which a great
quantity of scales were compactly imbricated. The opinion of M.
Brongniart that the _ Lepidostrobus_ is the fruit of _Lepidodendron_
has been confirmed, for these _strobili_ or fruits have been found
terminating the tip of a branch of a well-characterised _
Lepidodendron_ in Coalbrook Dale and elsewhere.
Fig. 457: a. Lepidostrobus ornatus; b. Portion of a section, showing
the large sporangia in their natural position, and each supported by
its bract or scale; c. Spores in these sporangia, highly magnified.
Fig. 458: Calamites Sucowii, common throughout Europe. Fig. 459: Stem
of Fig. 458, as retored by Dr. Dawson.
Equisetaceæ.—To this family belong two fossil genera of the coal,
_Equisetites_ and _Calamites._ The Calamites were evidently closely
related to the modern horse-tails (Equiseta) differing principally in
their great size, the want of sheaths at the joints, and some details
of fructification. They grew in dense brakes on sandy and muddy flats
in the manner of modern Equisetaceæ, and their remains are frequent in
the coal. Seven species of this plant occur in the great Nova Scotia
section before described, where the stems of some of them five inches
in diameter, and sometimes eight feet high, may be seen terminating
downward in a tapering root (see Fig. 460).
Fig. 460: Radical termination of a Calamite. Fig. 461: Asterophyllites
foliosus, Coal-measures, Newcastle.
Botanists are not yet agreed whether the _Asterophyllites,_ a species
of which is represented in Fig. 461, can form a separate genus from the
Calamite, from which, however, according to Dr. Dawson, its foliage is
distinguished by a true mid-rib, which is wanting in the leaves known
to belong to some Calamites.
Fig. 462: Annularia sphenophylloides.Fig. 463: Sphenophyllum erosum.
Figs. 462 and 463 represent leaves of _Annularia_ and _ Sphenophyllum,_
common in the coal, and believed by Mr. Carruthers to be leaves of
Calamites. Dr. Williamson, who has carefully studied the Calamites,
thinks that they had a fistular pith, exogenous woody stem, and thick
smooth bark, which last having always disappeared, leaves a fluted
stem, as represented in Fig. 459.
Sigillaria.—A large portion of the trees of the Carboniferous period
belonged to this genus, of which as many as 28 species are enumerated
as British. The structure, both internal and external, was very
peculiar, and, with reference to existing types, very anomalous. They
were formerly referred, by M. Ad. Brongniart, to ferns, which they
resemble in the scalariform texture of their vessels and, in some
degree, in the form of the cicatrices left by the base of the
leaf-stalks which have fallen off (see Fig. 464). But some of them are
ascertained to have had long linear leaves, quite unlike those of
ferns. They grew to a great height, from 30 to 60, or even 70 feet,
with regular cylindrical stems, and without branches, although some
species were dichotomous towards the top. Their fluted trunks, from one
to five feet in diameter, appear to have decayed more rapidly in the
interior than externally, so that they became hollow when standing; and
when thrown prostrate, they were squeezed down and flattened. Hence, we
find the bark of the two opposite sides (now converted into bright
shining coal) constitute two horizontal layers, one upon the other,
half an inch, or an inch, in their united thickness. These same trunks,
when they are placed obliquely or vertically to the planes of
stratification, retain their original rounded form, and are
uncompressed, the cylinder of bark having been filled with sand, which
now affords a cast of the interior.
Fig. 464: Sigillaria lævigata.
Dr. Hooker inclined to the belief that the _ Sigillariæ_ may have been
cryptogamous, though more highly developed than any flowerless plants
now living. Dr. Dawson having found in some species what he regards as
medullary rays, thinks with Brongniart that they have some relation to
gymnogens, while Mr. Carruthers leans to the opinion that they belong
to the Lycopodiaceæ.
_Stigmaria._—This fossil, the importance of which has already been
pointed out in p. 398, was originally conjectured to be an aquatic
plant. It is now ascertained to be the root of _Sigillaria._ The
connection of the roots with the stem, previously suspected, on
botanical grounds, by Brongniart, was first proved, by actual contact,
in the Lancashire coal-field, by Mr. Binney. The fact has lately been
shown, even more distinctly, by Mr. Richard Brown, in his description
of the _Stigmariæ_ occurring in the under-clays of the coal-seams of
the Island of Cape Breton, in Nova Scotia. In a specimen of one of
these, represented in Fig. 465, the spread of the roots was sixteen
feet, and some of them sent out rootlets, in all directions, into the
surrounding clay.
Fig. 465: Stigmaria attached to a trunk of Sigillaria.
In the sea-cliffs of the South Joggins in Nova Scotia, I examined
several erect _Sigillariæ,_ in company with Dr. Dawson, and we found
that from the lower extremities of the trunk they sent out _Stigmariæ_
as roots. All the stools of the fossil trees dug out by us divided into
four parts, and these again bifurcated, forming eight roots, which were
also dichotomous when traceable far enough. The cylindrical rootlets
formerly regarded as leaves are now shown by more perfect specimens to
have been attached to the root by fitting into deep cylindrical pits.
In the fossil there is rarely any trace of the form of these cavities,
in consequence of the shrinkage of the surrounding tissues. Where the
rootlets are removed, nothing remains on the surface of the Stigmaria
but rows of mammillated tubercles (see Figs. 466, 467), which have
formed the base of each rootlet.
Fig. 466: Stigmaria ficoides. Fig. 467: Surface of another individual
of same species, showing form of tubercles.
These protuberances may possibly indicate the place of a joint at the
lower extremity of the rootlet. Rows of these tubercles are arranged
spirally round each root, which have always a medullary axis and woody
system much resembling that of _Sigillaria,_ the structure of the
vessels being, like it, scalariform.
Coniferæ.—The coniferous trees of this period are referred to five
genera; the woody structure of some of them showing that they were
allied to the Araucarian division of pines, more than to any of our
common European firs. Some of their trunks exceeded forty-four feet in
height. Many, if not all of them, seem to have differed from living
_Coniferæ_ in having large piths; for Professor Williamson has
demonstrated the fossil of the coal-measures called _Sternbergia_ to be
the pith of these trees, or rather the cast of cavities formed by the
shrinking or partial absorption of the original medullary axis (see
Figs. 468, 469). This peculiar type of pith is observed in living
plants of very different families, such as the common Walnut and the
White Jasmine, in which the pith becomes so reduced as simply to form a
thin lining of the medullary cavity, across which transverse plates of
pith extend horizontally, so as to divide the cylindrical hollow into
discoid interspaces. When these interspaces have been filled up with
inorganic matter, they constitute an axis to which, before their true
nature was known, the provisional name of _Sternbergia_ (_d, d,_ Fig.
468) was given. In the above specimen the structure of the wood (_b,_
Figs. 468 and 469) is coniferous, and the fossil is referable to
Endlicher’s fossil genus _ Dadoxylon._
Fig. 468: Fragment of coniferous wood. Fig. 468: Fragment of coniferous
wood, _Dadoxylon_, of Endlicher, fractured longitudinally; from
Coalbrook Dale.
W.C. Williamson[3]
Fig. 469: Magnified portion of Fig. 468; transverse section.
The fossil named _Trigonocarpon_ (Figs. 470 and 471), formerly supposed
to be the fruit of a palm, may now, according to Dr. Hooker, be
referred, like the _Sternbergia,_ to the _ Coniferæ._ Its geological
importance is great, for so abundant is it in the coal-measures, that
in certain localities the fruit of some species may be procured by the
bushel; nor is there any part of the formation where they do not occur,
except the under-clays and limestone. The sandstone, ironstone, shales,
and coal itself, all contain them. Mr. Binney has at length found in
the clay-ironstone of Lancashire several specimens displaying
structure, and from these, says Dr. Hooker, we learn that the _
Trigonocarpon_ belonged to that large section of existing coniferous
plants which bear fleshy solitary fruits, and not cones. It resembled
very closely the fruit of the Chinese genus _ Salisburia,_ one of the
Yew tribe, or Taxoid conifers.
Fig. 470: Trigonocarpum ovatum.Fig. 471: Trigonocarpum olivæforme.
Fig. 472: Antholithes.
Angiosperms.—The curious fossils called _ Antholithes_ by Lindley have
usually been considered to be flower spikes, having what seems a calyx
and linear petals (see Fig. 472). Dr. Hooker, after seeing very perfect
specimens, also thought that they resembled the spike of a
highly-organised plant in full flower, such as one of the
_Bromeliaceæ,_ to which Professor Lindley had at first compared them.
Mr. Carruthers, who has lately examined a large series in different
museums, considers it to be a dicotyledonous angiosperm allied to _
Orobanche_ (broom-rape), which grew, not on the soil, but parasitically
on the trees of the coal forests.
In the coal-measures of Granton, near Edinburgh, a remarkable fossil
(Fig. 473) was found and described in 1840,[4] by Dr. Robert Paterson.
It was compressed between layers of bituminous shale, and consists of a
stem bearing a cylindrical spike, _a,_ which in the portion preserved
in the slate exhibits two subdivisions and part of a third. The spike
is covered on the exposed surface with the four-cleft calyces of the
flowers arranged in parallel rows. The stem shows, at _b,_ a little
below the spike, remains of a lateral appendage, which is supposed to
indicate the beginning of the spathe. The fossil has been referred to
the _ Aroidiæ,_ and there is every probability that it is a true member
of this order. There can at least be no doubt as to the high grade of
its organisation, and that it belongs to the monocotyledonous
angiosperms. Mr. Carruthers has carefully examined the original
specimen in the Botanical Museum, Edinburgh, and thinks it may have
been an epiphyte.
Fig. 473: Pothocites Grantonii.
Climate of the Coal Period.—As to the climate of the Coal, the Ferns
and the Coniferæ 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 coniferæ, 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 coniferæ 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.—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 _Palæozoic,_
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 (Figs. 474, 475)
may illustrate these types.
Fig. 474: Palæozoic type of lamelliferous cup-shaped Coral.
Vertical section of _Campophyllum flexuosum,_ (_Cyathophyllum,_
Goldfuss); from the Devonian of the Eifel. The lamellæ are seen around
the inside of the cup; the walls consist of cellular tissue; and large
transverse plates, called _ tubulæ,_ divide the interior into chambers.
Arrangement of the _lamellæ_ in _Polycoelia profunda,_ Germar, sp.;
from the Magnesian Limestone, Durham. This diagram shows the
quadripartite arrangement of the primary septa, characteristic of
palæozoic corals, there being four principal and eight intermediate
lamellæ, the whole number in this type being always a multiple of four.
_Stauria astræiformis,_ Milne Edwards. Young group, natural size. Upper
Silurian, Gothland. The lamellæ or septal system in each cup are
divided by four prominent ridges into four groups.
Fig. 475: Neozoic type of lamelliferous cup-shaped Coral.
_Parasmilia centralis,_ Mantell, sp. Vertical section. Upper Chalk,
Gravesend. In this type the lamellæ are massive, and extend to the axis
or columella composed of loose cellular tissue, without any transverse
plates like those in Fig. 474, _ a._
_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._
_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.
It will be seen that the more ancient corals have what is called a
quadripartite arrangement of the chief plates or _ lamellæ_—parts of
the skeleton which support the organs of reproduction. The number of
these lamellæ in the Palæozoic 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 Figs. 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 excepions to the
rule. Thus in the Lower Greensand _ Holocystis elegans_ (Ed. and H.)
and other forms have the Palæozoic type, and Dr. Duncan has shown to
what extent the Neozoic forms penetrate downward into the Carboniferous
and Devonian rocks.
Fig. 476: Lithostrotion basaltiforme. Fig. 477: Lonsdaleia floriformis.
From a great number of lamelliferous corals met with in the Mountain
Limestone, two species (Figs. 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,_[5] form a
group of rugose corals widely different from any that followed them.
Bryozoa and Crinoidea.—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 Figs. 478,
479), two genera, _Pentremites_ and _Codonaster,_ being peculiar to
this formation in Europe and North America.
Fig. 478: Cyathocrinus planus. Fig. 479: Cyathocrinus caryocrinoides.
Fig. 480: Palæchinus gigas.
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 Fig. 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 _Palæchinus_ (Fig. 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, _Archæocidaris,_ represents, in like manner, the
_Cidaris_ of the present seas.
Mollusca.—The British Carboniferous mollusca enumerated by Mr.
Etheridge[6] comprise 653 species referable to 86 genera, occurring
chiefly in the Mountain Limestone. 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_ (Fig. 481), and _P. scabriculus._ Large plaited
spirifers, as _Spirifera striata, S. rotundata,_ and _S. trigonalis_
(Fig. 482), also abound; and smooth species, such as _Spirifera glabra_
(Fig. 483), with its numerous varieties.
Fig. 481: Productus semireticulatus. Fig. 482: Spirifera trigonalis.
Fig. 483: Spirifera glabra.
Fig. 484: Terebratula hastata. Fig. 485: Aviculopecten sublobatus. Fig.
486: Pleurotomaria carinata.
Among the brachiopoda, _Terebratula hastata_ (Fig. 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_ (Fig. 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_ (Fig. 486), which displays wavy blotches,
resembling the colouring in many recent trochidæ.
Fig. 487: Euomphalus pentagulatus.
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
Palæozoic epoch. _Euomphalus_ is a characteristic univalve shell of
this period. In the interior it is divided into chambers (Fig. 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.
More than twenty species of the genus _Bellerophon_ (see Fig. 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 Atlantidæ, partly allied to the Glass-Shell,
_Carinaria_; but by some few it is thought to be a simple form of
Cephalopod.
Fig. 488: Bellerophon costatus.
Fig. 489: Portion of Orthoxeras laterale. Fig. 490: Goniatites
crenistra.
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 (Fig. 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 Fig. 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.—The distribution of these is singularly partial; so much
so, that M. De Koninck of Liége, the eminent palæontologist, 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 Fig. 261),
massive palatal teeth fitted for grinding. (See Figs. 491, 492.)
Fig. 491: Psammodus porosus.
Fig. 492: Cochliodus controtus.
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.—In the upper part of the Mountain Limestone group in the
S.W. 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_ (Fig. 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 (Fig. 493) is characteristic of the
Mountain Limestone in the United States, Arctic America, Russia, and
Asia Minor, but is also known in the Permian.
Fig. 493: Fusulina cylindrica.
[1] For botanical nomenclature see p. 304.
[2] Sir C. Bunbury, Quart. Geol. Journ., vol. ii, 1845.
[3] Manchester Phil. Mem., vol. ix, 1851.
[4] Trans. of Bot. Soc. of Edinburgh, vol. i, 1844.
[5] For figures of these corals, see Palæontographical Society’s
Monographs, 1852.
[6] Quart. Geol. Journ., vol. xxiii, p. 674, 1867.
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.—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 Fig. 494).
Fig. 494: Anodonta Jukesii.
Fig. 495: Bifurcating branch of Lepidendron Griffithsii.
Fig. 496: Palæopteris Hibernia.
In the same formation are found the fern (Fig. 496) and the _
Lepidodendron_ (Fig. 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° 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. Palæontologists 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.
Fig. 497: Scale of Holoptychius nobilissimus.
Outliers of the Upper “Old Red” occur unconformably on older members of
the group, and the formation represented at Whiteness, near Arbroath,
_a,_ Fig. 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 Fig. 498 of this fish and also Fig. 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.[1] The number of fish obtained from the
British Upper Old Red Sandstone amounts to fifteen species referred to
eleven genera.
Fig. 498: Holoptychius, as restored by Professor Huxley.
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.[2]
_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.
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 _ Crossopterygidæ_,[3] 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,_ Fig. 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_ (Fig. 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.
Fig. 499: Polypterus. Living in the Nile and other rivers.
Fig. 500: Restoration of Osteolepis.
Among the “fringe-finned” Ganoids we find some with rhomboidal scales,
such as _Osteolepis,_ Fig. 500; others with cycloidal scales, as
_Holoptychius,_ before mentioned (see Fig. 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. Günther, and of which
more than 6000 are now preserved in the British Museum, they probably
constitute no more than nine.
Fig. 501: Pterichthys. Upper side, showing mouth.
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.
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._
Fig. 502: Cephalapsis Lyellii.
Lower Old Red Sandstone.—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 Fig. 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 Acanthodidæ, the spines, scales, and other
remains of which have been detected in the grey flaggy sandstones.[4]
Fig. 503: Pteygotus anglicus.
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 Grès Rouge.”
Fig. 504: Pterygotus anglicus. Ventral aspect.
Carapace, showing the large sessile eyes at the anterior angles.
The _metastoma_ or post-oral plate (serving the office of a lower lip).
Chelate appendages (antennules).
First pair of simple palpi (antennæ).
Second pair of simple palpi (mandibles).
Third pair of simple palpi (first maxillæ).
Pair of swimming feet with their broad basal joints, whose serrated
edges serve the office of maxillæ.
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.)
A restoration in correct proportion to the size of the fragments of _P.
anglicus_ (Fig. 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 1½ foot across its carapace, and is
three feet in length.
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
Figs. 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.
Fig. 505: Parka decipiens. In sandstone of lower beds of Old Red, Ley’s
Mill, Forfarshire. Fig. 506: Parka decipiens. In shale of Lower Old
Red, Park Hill, Fife.
Fig. 507: Shale of Old Red Sandstone. Forfarshire. With impression of
plants and eggs of Crustaceans.
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 palæontological 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.
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
cucullæa (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.).
The above table 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[5] 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. 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.
Fig. 508: Spirifera disjuncta. Fig. 509: Phacops latifrons.
Upper Devonian Rocks.—The slates and sandstones of Barnstaple (_a_ and
_b_ of the preceding section) contain the shell _Spirifera disjuncta,_
Sowerby (S. Verneuilii, Murch.), (see Fig. 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 Fig. 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_ (Fig. 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_ (Fig. 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.—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_ (Fig. 513), one of the commonest of all the Devonshire
fossils. The _Cyathophyllum cæspitosum_ (Fig. 514) and _Heliolites
pyriformis_ (Fig. 512) are species peculiar to this formation.
Fig. 510: Clymenia linearis. Fig. 511: Cypridina serrato-striata.
Fig. 512: Heliolites porosa.
Fig. 513: Favosites cervicornis. Fig. 514: Cyathophyllum cæspitosum.
Fig. 515: Stringocephalus Burtini. Fig. 516: Uncites Gryphus.
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_ (Fig. 515) and _Uncites
Gryphus_ (Fig. 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.
Fig. 517: Megalodon cucullatus.
The only two species of Brachiopoda common to the Silurian and Devonian
formations are _Atrypa reticularis_ (Fig. 532), which seems to have
been a cosmopolite species, and _Strophomena rhomboidalis._
Among the peculiar lamellibranchiate bivalves common to the Plymouth
limestone of Devonshire and the Continent, we find the _ Megalodon_
(Fig. 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_ (Fig. 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_
(Fig. 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, p. 362), to identify this
middle marine Devonian with the Old Red Sandstone of Britain and
Russia.
Fig. 518: Conularia ornata.Fig. 519: Bronteus flabellifer.
Fig. 520: Calceola sandalina.
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_ (Fig. 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 Fig. 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.—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
p. 449, 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.
Fig. 521: Spirifora mucronata.
Fig. 522: Homalonotus armatus.
Among the Trilobites of this era several large species of _
Homalonotus_ (Fig. 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.[6] 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.
Fig. 523: Psilophyton princeps.
Devonian Strata in the United States and Canada.—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 Fig. 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 Göppert, 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,[7]
comprising _Coniferæ, Sigillariæ, 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,_
Fig. 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.
[1] Siluria, 4th ed., p. 265.
[2] Siluria, 4th ed., p. 258.
[3] Abridged from _crossotos,_ a fringe, and _ pteryx,_ a fin.
[4] Powrie, Geol. Quart. Journ., vol. xx, p. 417.
[5] Quart. Geol. Journ., vol. xxiii., 1867.
[6] Murchison’s Siluria, p. 329.
[7] Quart. Geol. Journ., vol. xv, p. 477, 1859; also vol. xviii, p.
296, 1862.
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. — Cystideæ. — 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 Palæozoic 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. The following table 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.
UPPER SILURIAN ROCKS. Thickness
in feet 1. LUDLOW FORMATION:
_a._ Upper Ludlow beds 780 _b._ Lower Ludlow beds: 1,050 2.
WENLOCK FORMATION:
_a._ Wenlock limestone and shale above 4,000 _b._ Woolhope
limestone and shale, and Denbighshire grits: 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 above table, 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
Fig. 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.[1] 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.
Fig. 524: Onchus tenuistriatus. Fig. 525: Shagreen-scales of a placoid
fish, Thelodus parvidens.
Most of the fish have been referred by Agassiz to his placoid order,
some of them to the genus Onchus, to which the spine (Fig. 524) and the
minute scales (Fig. 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.
Fig. 526: Plectrodus mirabilis.
The jaw and teeth of another predaceous genus (Fig. 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.
Fig. 527: Orthis elegantula.Fig. 528: Rhynchonella navicula.
_Grey Sandstone and Mudstone, etc._—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 aon page 459, _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_ (Fig. 528), which is
common to this bed and the Lower Ludlow. As usual in Palæozoic strata
older than the coal, the brachiopodous mollusca greatly outnumber the
lamellibranchiate (see p. 470); 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 _ Palæarca._
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.
Fig. 529: Pentamerus Knightii.
_b._ Lower Ludlow Beds.—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 (Fig.
529), also found in the Lower Ludlow and Wenlock shale. This genus of
brachiopoda was first found in Silurian strata, and is exclusively a
palæozoic 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.
Fig. 530: Lingula Lewisii.
Three other abundant shells in the Aymestry limestone are, first,
_Lingula Lewisii_ (Fig. 530); second, _Rhynchonella Wilsoni,_ Sowerby
(Fig. 531), which is also common to the Lower Ludlow and Wenlock
limestone; third, _Atrypa reticularis,_ Linn. (Fig. 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.
Fig. 531: Rhynchonella (Terebratula) Wilsoni.
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.
Fig. 532: Atrypa reticularis. 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 Figs.
533, 534). The latter is partly straight and partly convoluted in a
very flat spire. The _Orthoceras Ludense_ (Fig. 535), as well as the
cephalopod last mentioned, occurs in this member of the species.
Fig. 533: Phragmoceras ventricosum.
A species of Graptolite, _G. priodon,_ Bronn (Fig. 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 _Asteriadæ_ and _ Ophiuridæ._
Fig. 534: Lituites (Trochoceras) giganteus. Fig. 535: Fragment of
Orthoceras Ludense.
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.
_Dates of the Discovery of different Classes of Fossil Vertebrata;
showing the gradual progress made in tracing them to rocks of higher
antiquity._
Year Formations Geographical localities
Mammalia 1798 Upper Eocene Paris (Gypsum of Montmartre).1
1818 Lower Oolite Stonesfield.2 1847 Upper
Trias Stuttgart.3 Aves 1782 Upper Eocene Paris (Gypsum
of Montmartre).4 1839 Lower Eocene Isle of Sheppey (London
Clay).5 1854 Lower Eocene Woolwich Beds.6 1855 Lower
Eocene Mendon (Plastic Clay).7 1858 Chloritic Series, or
Upper Greensand Cambridge.8 1863 Upper
Oolite Solenhofen.9 Reptilia 1710 Permian (or
Zechstein) Thuringia.10 1844 Carboniferous Saarbrück,
near Trèves.11 Pisces 1709 Permian (or
Kupferschiefer) Thuringia.12 1793 Carboniferous (Mountain
Limestone) Glasgow.13 1828 Devonian Caithness.14
1840 Upper Ludlow Ludlow.15 1859 Lower
Ludlow Leintwardine.16
1. George Cuvier, Bulletin Soc. Philom. xx.
2. In 1818, Cuvier, visiting the Museum of Oxford, decided on the
mammalian character of a jaw from Stonesfield. See also p. 347.
3. Prof. Plieninger. See p. 368.
4. Cuvier, Ossemens Foss. Art. “Oiseaux.”
5. Prof. Owen, Geol. Trans., 2nd series, vol. vi, p. 203, 1839.
6. Upper part of the Woolwich beds. Prestwich, Quart. Geol. Journ.,
vol. x, p. 157.
7. _Gastornis Parisiensis._ Owen, Quart. Geol. Journ., vol. xii, p.
204, 1856.
8. Coprolitic bed, in the Upper Greensand. See p. 299.
9. The _Archæopteryx macrura,_ Owen. See p. 338.
10. The fossil monitor of Thuringia (_Protosaurus Speneri,_ V. Meyer)
was figured by Spener of Berlin in 1810. (Miscel. Berlin.)
11. See p. 406.
12. Memorabilia Saxoniæ Subterr., Leipsic, 1709.
13. History of Rutherglen by Rev. David Ure, 1793.
14. Sedgwick and Murchison, Geol. Trans., 2nd series, vol. ii, p. 141,
1828.
15. Sir R. Murchison. See p. 459.
16. See p. 461.
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 the preceding Table 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 (see Table, p. 458) into Wenlock limestone, Wenlock shale,
and Woolhope limestone and Denbighshire grits.
Fig. 536: Halysites catenularius.
_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[2] Sometimes in the Malvern Hills this limestone,
according to Professor Phillips, is oolitic.
Fig. 537: Favosites Gothlandica.
Among the corals, in which this formation is so rich, 53 species being
known, the “chain-coral,” _Halysites catenularius_ (Fig. 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_ (Fig. 537), is also met with in profusion in
large hemispherical masses, which break up into columnar and prismatic
fragments, like that here figured (Fig. 537, _b_). Another common form
in the Wenlock limestone is the _Omphyma turbinatum_ (Fig. 538), which,
like many of its modern companions, reminds us of some cup-corals; but
all the Silurian genera belong to the palæozoic type before mentioned
(p. 432), exhibiting the quadripartite arrangement of the septalamellæ
within the cup.
Fig. 538: Omphyma turbinatum.Fig. 539: Pseudocrinites bifasciatus.
Among the numerous Crinoids, several peculiar species of _
Cyathocrinus_ (for genus see Figs. 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,[3] as
represented in Fig. 539.
Fig. 540: Strophomena (Leptæna) depressa.
The Brachiopoda are, many of them, of the same species as those of the
Aymestry limestone; as, for example, _Atrypa reticularis_ (Fig. 532),
and _Strophomena depressa_ (Fig. 540); but the latter species ranges
also from the Ludlow rocks, through the Wenlock shale, to the Caradoc
Sandstone.
Fig. 541: Calymene Blumenbachii.
The crustaceans are represented almost exclusively by Trilobites, which
are very conspicuous, 22 being peculiar. The _ Calymene Blumenbachii_
(Fig. 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 Fig. 542), and
this is conspicuous for its large size and flattened form.
_Sphærexochus mirus_ (Fig. 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 Fig. 544), is very characteristic
of this division of the Silurian series.
Fig. 542: Phacops (Asaphus) caudatus.Fig. 543: Sphærexochus mirus.
_Wenlock Shale._—This, 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.
Fig. 544: Homalonotus delphinocephalus.
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 (p. 463) 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
(p. 474).
Fig. 545: Graptolithus priodon.
_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 _Illænus Barriensis, Homalonotus
delphinocephalus_ (Fig. 544), _Strophomena imbrex,_ and _Rhynchonella
Wilsoni_ (Fig. 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._—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[4] to be at the base of the Upper Silurian
proper. The more calcareous portions of the rock have been called the
Pentamerus limestone, because _Pentamerus oblongus_ (Fig. 546) is very
abundant in them. It is usually accompanied by _P. (Stricklandinia)
lirata_ (Fig. 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.
Fig. 546: Pentamerus oblongus.
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 _Illænus_ and _Calymene_; Brachiopods of the genera _Orthis,
Atrypa, Leptæna, Pentamerus, Strophomena,_ and others; Gasteropods of
the genera _Turbo, Murchisonia_ (for genus, see Fig. 567), and
_Bellerophon_; and Pteropods of the genus _Conularia._ The Brachiopods,
of which there are 66 species, are almost all Upper Silurian.
Fig. 547: Stricklandinia (Pentamerus) lirata. Fig. 548: Tentaculites
annulatus.
Among the fossils of the May-Hill shelly sandstone at Malvern,
_Tentaculites annulatus_ (Fig. 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_ (Fig. 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;
second, the Llandeilo flags; and, third, the Arenig or Lower Llandeilo
formation.
Bala and Caradoc Beds.—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 Fig. 553), which ranges from the base to the summit of
the formation, usually accompanied by _Strophomena grandis_ (see Fig.
551), and _Orthis vespertilio_ (Fig. 550), with many other fossils.
Fig. 549: Orthis tricenaria. Fig. 550: Orthis vespertilio. Fig. 551:
Orthis (Strophomena) grandis.
_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 palæozoic 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.
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 Figs. 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.
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 _Palæaster,_ are found; the fossil specimen of the
latter (Fig. 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 _Cystideæ._ They are the _Sphæronites_ of
old authors, and were considered by Professor E. Forbes as intermediate
between the crinoids and echinoderms. The _Echinosphæronite_ here
represented (Fig. 555) is characteristic of the Caradoc beds in Wales,
and of their equivalents in Sweden and Russia.
Fig. 552: Young individuals of Trinucleus concentricus. Fig. 553:
Trinucleus concentricus.
Fig. 554: Palæaster asperimus.
Fig. 555: Echinosphæronites ballicus.
With it have been found several other genera of the same family, such
as _Sphæronites, Hemicosmites,_ etc. Among the mollusca are Pteropods
of the genus _Conularia_ of large size (for genus, see Fig. 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.
Fig. 556: Didymograpsus (Graptolites) Murchisonii.
Llandeilo Flags.—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.
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
(Fig. 557), are conspicuous.
Fig. 557: Diplograpsus pristis. Fig. 558: Rastrites peregrinus.
Fig. 559: Diplograpsus folium.
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.
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 Fig. 560).
Fig. 560: Orthoceras duplex.
Among other Cephalopods in the Llandeilo flags is Cyrtoceras; in the
same beds also are found Bellerophon (see Fig. 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_ (Fig. 561), _Ogygia_ (Fig. 562), and _Trinucleus_
(Figs. 552 and 553) form a marked feature of the rich and varied
Trilobitic fauna of this age.
Fig. 561: Asaphus tyrannus. Fig. 562: Ogygia Buchii.
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.
Fig. 563: Arenicolites linearis.
Arenig or Stiper-Stones Group _(Lower Llandeilo of Murchison)._—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 Fig. 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 (Fig.
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” (p. 52, 1859) to be
quite distinct from the fossils of the overlying Llandeilo flags. Among
these the _Obolella plumbea, Æglina binodosa, Ogygia Selwynii,_ and
_Didymograpsus geminus_ (Fig. 564), and _D. Hirundo,_ are
characteristic.
Fig. 564: Didymograpsus geminus.
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,[5] 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.
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_ _Shells of the lowest known
Fossiliferous Beds in Russia._
Fig. 565: Siphonotreta unguiculata. Fig. 566: Obolus Apollinis.
or Ungulite of Pander, and a _Siphonotreta_ (Figs. 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.—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 the table
below have been adopted.
_Subdivisions of the Silurian Strata of New York.
(Strata below the Oriskany sandstone or base of the Devonian.)_
New York Names British equivalents 1. Upper Pentamerus Limestone
2. Encrinal Limestone
3. Delthyris Shaly Limestone
4. Pentamerus and Tentaculite Limestones
5. Water Lime Group
6. Onondaga Salt Group
7. Niagara Group Upper Silurian (or Ludlow
and Wenlock formations 8. Clinton Group
9. Medina Sandstone
10. Oneida Conglomerate
11. Gray Sandstone Beds of Passage, Llandovery Group. 12. Hudson
River Group
13. Trenton Limestone
14. Black-River Limestone
15. Bird’s-eye Limestone
16. Chazy Limestone
17. Calciferous Sandstone Lower Silurian (or Caradoc and Bala,
Llandeilo and Arenig Formations).
In the second column of the same table I have added the supposed
British equivalents. All Palæontologists, 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_
(Fig. 544), with several other trilobites; _Rhynchonella Wilsoni,_ Fig.
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.
Fig. 567: Murchisonia gracilis.
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.
The Hudson River Group, and the Trenton Limestone, agree
palæontologically with the Caradoc or Bala group, containing in common
with them several species of trilobites, such as _ Asaphus (Isotelus)
gigas, Trinucleus concentricus_ (Fig. 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,_ Fig. 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,[6] 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 ”primæval seas” of one uniform specific fauna
was quite unfounded, geographical provinces having evidently existed in
the oldest as in the most modern times.
[1] Murchison’s Siluria, p. 140.
[2] Murchison’s Siluria, chap. vi.
[3] E. Forbes, Mem. Geol. Surv., vol. ii, p. 496.
[4] Quart. Geol. Journ., vol. iv, p. 215, 1853.
[5] Trans. Brit. Assoc., 1866. Proc. Liverpool Geol. Soc., 1869.
[6] Quart. Geol. Journ., vol. iv.
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 palæontological 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.
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 the table below. 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.
The following table will show 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
Tremadoc Slates.—The Tremadoc slates of Sedgwick are more than 1000
feet in thickness, and consist of dark earthy slates occurring near the
little town of Tremadoc, situated on the north side of Cardigan Bay, in
Carnarvonshire. These slates were first examined by Sedgwick in 1831,
and were re-examined by him and described in 1846,[1] after some
fossils had been found in the underlying Lingula flags by Mr. Davis.
The inferiority in position of these Lingula flags to the Tremadoc beds
was at the same time established. The overlying Tremadoc beds were
traced by their pisolitic ore from Tremadoc to Dolgelly. No fossils
proper to the Tremadoc slates were then observed, but subsequently,
thirty-six species of all classes have been found in them, thanks to
the researches of Messrs. Salter, Homfray, and Ash. We have already
seen that in the Arenig or Stiper-Stones group, where the species are
distinct, the genera agree with Silurian types; but in these Tremadoc
slates, where the species are also peculiar, there is about an equal
admixture of Silurian types with those which Barrande has termed
“primordial.” Here, therefore, it may truly be said that we are
entering upon a new domain of life in our retrospective survey of the
past. The trilobites of new species, but of Lower Silurian genera,
belong to _Ogygia, Asaphus,_ and _Cheirurus_; whereas those belonging
to primordial types, or Barrande’s first fauna as well as to the
Lingula flags of Wales, comprise _Dikelocephalus, Conocoryphe_ (for
genera see Fig. 577 and 581),[2] _Olenus,_ and _Angelina._
Fig. 568: Theca (Cleidotheca operculata.
In the Tremadoc slates are found _Bellerophon, Orthoceras,_ and
_Cyrtoceras,_ all specifically distinct from Lower Silurian fossils of
the same genera: the Pteropods _Theca_ (Fig. 568) and _Conularia_ range
throughout these slates; there are no Graptolites. The _Lingula
(Lingulella) Davisii_ ranges from the top to the bottom of the
formation, and links it with the zone next to be described. The
Tremadoc slates are very local, and seem to be confined to a small part
of North Wales; and Professor Ramsay supposes them to lie unconformably
on the Lingula flags, and that a long interval of time elapsed between
these formations. Cephalopoda have not yet been found lower than this
group, but it will be observed that they occur here associated with
genera of Trilobites considered by Barrande as characteristically
Primordial, some of which belong to all the divisions of the British
Cambrian about to be mentioned. This renders the absence of cephalopoda
of less importance as bearing on the theory of development.
Lingula Flags.—Next below the Tremadoc slates in North Wales lie
micaceous flagstones and slates, in which, in 1846, Mr. E. Davis
discovered the _Lingula (Lingulella),_ Fig. 570, named after him, and
from which was derived the name of Lingula flags. These beds, which are
palæontologically the equivalents of Barrande’s primordial zone, are
represented by more than 5000 feet of strata, and have been studied
chiefly in the neighbourhood of Dolgelly, Ffestiniog, and Portmadoc in
North Wales, and at St. David’s in South Wales. They have yielded about
forty species of fossils, of which six only are common to the overlying
Tremadoc rocks, but the two formations are closely allied by having
several characteristic “primordial” genera in common. _Dikelocephalus,
Olenus_ (Fig. 571), and _Conocoryphe_ are prominent forms, as is also
_Hymenocaris_ (Fig. 569), a genus of phyllopod crustacean entirely
confined to the Lingula Flags. According to Mr. Belt, who has devoted
much attention to these beds, there are already palæontological data
for subdividing the Lingula Flags into three sections.[3]
“Lingula Flags” of Dolgelly, and Ffestiniog; N. Wales.
In Merionethshire, according to Professor Ramsay, the Lingula Flags
attain their greatest development; in Carnarvonshire they thin out so
as to have lost two-thirds of their thickness in eleven miles, while in
Anglesea and on the Menai Straits both they and the Tremadoc beds are
entirely absent, and the Lower Silurian rests directly on Lower
Cambrian strata.
LOWER CAMBRIAN.
Menevian Beds.—Immediately beneath the Lingula Flags there occurs a
series of dark grey and black flags and slates alternating at the upper
part with some beds of sandstone, the whole reaching a thickness of
from 500 to 600 feet. These beds were formerly classed, on purely
lithological grounds, as the base of the Lingula Flags, but Messrs.
Hicks and Salter, to whose exertions we owe almost all our knowledge of
the fossils, have pointed out[4] that the most characteristic genera
found in them are quite unknown in the Lingula Flags, while they
possess many of the strictly Lower Cambrian genera, such as _
Microdiscus_ and _Paradoxides._ They therefore proposed to place them,
and it seems to me with good reason, at the top of the Lower Cambrian
under the term “Menevian,” Menevia being the classical name of St.
David’s. The beds are well exhibited in the neighbourhood of St.
David’s in South Wales, and near Dolgelly and Maentwrog in North Wales.
They are the equivalents of the lowest part of Barrande’s Primordial
Zone (Étage C). More than forty species have been found in them, and
the group is altogether very rich in fossils for so early a period.