[ 458 ]
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.
a. Upper Ludlow
Lower Ludlow beds:
a. Wenlock limestone and
Woolhope limestone and shale, and Denbighshire grits:
(Beds of passage between Upper and
a. Upper Llandovery
|1. BALA AND CARADOC
BEDS, including volcanic rocks:
|2. LLANDEILO FLAGS,
including volcanic rocks:
|3. ARENIG OR
STIPER-STONES GROUP, including
[ 459 ]
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
[ 460 ]
intervening strata full of Upper Ludlow fossils.* 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.
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.
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.
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
* Murchison’s Siluria, p. 140.
[ 461 ]
Pterinea, Cardiola, Ctenodonta (sub-genus of
Nucula), Orthonota, Modiolopsis, and
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
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
[ 462 ]
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.
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.
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.
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
[ 463 ]
Orthoceras Ludense (Fig. 535), as well as the cephalopod
last mentioned, occurs in this member of the species.
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
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
When we reflect on the hundreds of Mollusks, Echinoderms,
[ 464 ]
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
Dates of the Discovery of different Classes of Fossil
Vertebrata; showing the gradual progress made in tracing them to
rocks of higher antiquity.
||Paris (Gypsum of Montmartre).1
||Paris (Gypsum of Montmartre).4
||Isle of Sheppey (London Clay).5
||Mendon (Plastic Clay).7
||Chloritic Series, or Upper Greensand
||Permian (or Zechstein)
||Permian (or Kupferschiefer)
||Carboniferous (Mountain Limestone)
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.
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.
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.
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
[ 465 ]
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.
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* Sometimes in the Malvern Hills this limestone,
according to Professor Phillips, is oolitic.
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
* Murchison’s Siluria, chap. vi.
[ 466 ]
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.
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,*
as represented in Fig. 539.
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.
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
* E. Forbes, Mem. Geol. Surv., vol. ii, p.
[ 467 ]
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.
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.
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).
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
[ 468 ]
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
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
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
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
[ 469 ]
Sedgwick* 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.
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.
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
* Quart. Geol. Journ., vol. iv, p. 215, 1853.
[ 470 ]
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
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.
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
[ 471 ]
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
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
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
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
[ 472 ]
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.
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
[ 473 ]
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
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
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
[ 474 ]
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.
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).
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
[ 475 ]
and Trinucleus (Figs.
552 and 553) form a marked feature of the rich and varied
Trilobitic fauna of this age.
Beneath the black slates above described of the Llandeilo
formation, Graptolites are still found in great variety and
abundance, and the characteristic genera of shells and trilobites
of the Lower Silurian rocks are still traceable downward, in
Shropshire, Cumberland, and North and South Wales, through a vast
depth of shaly beds, in some districts interstratified with
trappean formations of contemporaneous origin; these consist of
tuffs and lavas, the tuffs being formed of such materials as are
ejected from craters and deposited immediately on the bed of the
ocean, or washed into it from the land. According to Professor
Ramsay, their thickness is about 3300 feet in North Wales,
including those of the Lower Llandeilo. The lavas are feldspathic,
and of porphyritic structure, and, according to the same authority,
of an aggregate thickness of 2500 feet.
Arenig or Stiper-Stones Group (Lower Llandeilo of
Murchison).—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
[ 476 ]
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.
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,*
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
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
* Trans. Brit. Assoc., 1866. Proc. Liverpool Geol.
[ 477 ]
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
[ 478 ]
Shells of the lowest known Fossiliferous Beds in
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
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
|New York Names
| 1. Upper Pentamerus
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
|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
Llandeilo and Arenig Formations).
[ 479 ]
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
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
Mr. D. Sharpe, in his report on the mollusca collected by me
from these strata in North America,* has concluded that the number
of species common to the Silurian rocks
* Quart. Geol. Journ., vol. iv.
[ 480 ]
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.