Sir Charles Lyell

Silurian strata formerly called transition — Term grauwackÉ — Subdivisions of Upper and Lower Silurian — Ludlow formation and fossils — Wenlock formation, corals and shells — Caradoc and Llandeilo beds — Graptolites — Lingula — Trilobites — CystideÆ — Vast thickness of Silurian strata in North Wales — Unconformability of Caradoc sandstone — Silurian strata of the United States — Amount of specific agreement of fossils with those of Europe — Great number of brachiopods — Deep-sea origin of Silurian strata — Absence of fluviatile formations — Mineral character of the most ancient fossiliferous rocks.

We come next in the descending order to the most ancient of the primary fossiliferous rocks, that series which comprises the greater part of the strata formerly called "transition" by Werner, for reasons explained in Chap. VIII., pp. 91 and 92. Geologists have also applied to these older strata the general name of "grauwackÉ," by which the German miners designate a particular variety of sandstone, usually an aggregate of small fragments of quartz, flinty slate (or Lydian stone), and clay-slate cemented together by argillaceous matter. Far too much importance has been attached to this kind of rock, as if it belonged to a certain epoch in the earth's history, whereas a similar sandstone or grit is found sometimes in the Old Red, and in the Millstone Grit of the Coal, and sometimes in certain Cretaceous and even Eocene formations in the Alps.

The name of Silurian was first proposed by Sir Roderick Murchison, for a series of fossiliferous strata lying below the Old Red Sandstone, and occupying that part of Wales and some contiguous counties of England, which once constituted the kingdom of the Silures, a tribe of ancient Britons. The strata have been divided into Upper and Lower Silurian, and these again in the region alluded to admit of several well-marked subdivisions, all of them explained in the following table.

UPPER SILURIAN ROCKS.

Prevailing Lithological characters.

Thickness in Feet.

Organic Remains.

1. Ludlow formation

Tilestones.

Finely laminated reddish and green sandstones and shales.

800?

Marine mollusca of almost every order, the Brachiopoda most abundant. Serpula, Corals, Sauroid fish, Fuci.

Upper Ludlow.

Micaceous grey sandstone.

2000

Aymestry limestone.

Argillaceous limestone.

Lower Ludlow.

Shale, with concretions of limestone.

2. Wenlock formation.

Wenlock limestone.

Concretionary limestone.

1800

Marine mollusca of various orders as before, Crustaceans of the Trilobite family. Oldest bones of fish yet known.

Wenlock shale.

Argillaceous shale.

LOWER SILURIAN ROCKS.

3. Caradoc formation.

Caradoc sandstones.

Flags of shelly limestone and sandstone, thick bedded white freestone.

2500

Crinoidea, Corals, Mollusca, chiefly Brachiopoda, Trilobites.

4. Llandeilo formation.

Llandeilo flags.

Dark coloured calcareous flags.

1200

Mollusca, Trilobites.

UPPER SILURIAN ROCKS.

Ludlow formation.—This member of the Upper Silurian group, as will be seen by the above table, is of great thickness, and subdivided into four parts,—the Tilestone, the Upper and Lower Ludlow, and the intervening Aymestry limestone. Each of these may be distinguished near the town of Ludlow, and at other places in Shropshire and Herefordshire, by peculiar organic remains.

1. Tilestones.—This uppermost division was originally classed by Sir R. Murchison with the Old Red Sandstone, because they decompose into a red soil throughout the Silurian region. At the same time he regarded the tilestones as a transition group forming a passage from Silurian to Old Red. It is now ascertained that the fossils agree in great part specifically, and in general character entirely, with those of the succeeding formation.

2. Upper Ludlow.—The next division, called the Upper Ludlow, consists of grey calcareous sandstone, decomposing into soft mud, and contains, among other shells, the Lingula cornea, which is common to it and the lowest, or tilestone beds of the Old Red. But the Orthis orbicularis is peculiar to the Upper Ludlow, and very common; and the lowest or mudstone beds, are loaded for a thickness of 30 feet with Terebratula navicula (fig. 410.), in vast numbers. Among the cephalopodous mollusca occur the genera Bellerophon and Orthoceras, and among the crustacea the Homalonotus (fig. 418. p. 354.). A coral called Favosites polymorpha, Goldf. (fig. 401. p. 346.) is found both in this subdivision and in the Devonian system.

Fig. 409.

Orthis orbicularis, J. Sow. Delbury. Upper Ludlow.

Fig. 410.

Terebratula navicula, J. Sow. Aymestry limestone; also in Upper and Lower Ludlow.

Among the fossil shells are species of LeptÆna, Orthis, Terebratula, Avicula, Trochus, Orthoceras, Bellerophon, and others.[352-A]

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 "mudstones." In these shales many zoophytes are found enveloped 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.

The scales, spines (ichthyodorulites), jaws, and teeth of fish of the genera Onchus, Plectrodus, and others of the same family, have been met with in the Upper Ludlow rocks.

Fig. 411.

Pentamerus Knightii, Sow. Aymestry.

• a. view of both valves united.
• b. longitudinal section through both valves, showing the central plate or septum; half nat. size.

3. Aymestry limestone.—The next group is a subcrystalline and argillaceous limestone, which is in some places 50 feet thick, and distinguished around Aymestry by the abundance of Pentamerus Knightii, Sow. (fig. 411.), also found in the Lower Ludlow. This genus of brachiopoda has only been found in the Silurian strata. The name was derived from pe?te, pente, five, and e???, 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; but neither the structure of this shell, nor the connection of the animal with its several parts, are as yet understood. 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.

Fig. 412.

Lingula Lewisii, J. Sow. Abberley Hills.

Three other abundant shells in the Aymestry limestone are, 1st, Lingula Lewisii (fig. 412.); 2d, Terebratula Wilsoni, Sow. (fig. 413.), which is also common to the Lower Ludlow and Wenlock limestone; 3d, Atrypa reticularis, Lin. (fig. 414.), which has a very wide range, being found in every part of the Silurian system, except the Llandeilo flags.

Fig. 413.

Terebratula Wilsoni, Sow. Aymestry.

Fig. 414.

Atrypa reticularis. Linn. Syn. Terebratula affinis, Min. Con. Aymestry.

• a. upper valve.
• b. lower.
• c. anterior margin of the valves.

4. Lower Ludlow shale.—A dark grey argillaceous deposit, containing, among other fossils, the new genera of chambered shells, the Phragmoceras of Broderip, and the Lituites of Breyn (see figs. 415, 416.). The latter is partly straight and partly convoluted, nearly as in Spirula.

Fig. 415.

Phragmoceras ventricosum, J. Sow. (Orthoceras ventricosum, Stein.) Aymestry; ¹/₄ nat. size.

Fig. 416.

Lituites giganteus, J. Sow. Near Ludlow; also in the Aymestry and Wenlock limestones; ¹/₄ nat. size.

Fig. 417.

• a. Fragment of Orthoceras Ludense, J. Sow.
• b. Polished section, showing siphuncle. Ludlow.

The Orthoceras Ludense (fig. 417.), as well as the shell last mentioned, is peculiar to this member of the series. The Homalonotus delphinocephalus (fig. 418.) is common to this division and to the Wenlock limestone. This crustacean belongs to a group of trilobites which has been met with in the Silurian rocks only, and in which the tripartite character of the dorsal crust is almost lost.

Fig. 418.

Homalonotus delphinocephalus, KÖnig.[354-A] Dudley Castle; ¹/₂ nat. size.

A species of Graptolite, G. Ludensis, Murch. (fig. 419.), a form of zoophyte which has not yet been met with in strata newer than the Silurian, occurs in the Lower Ludlow.

Wenlock formation.—We next come to the Wenlock formation, which has been divided (see Table, p. 351.) into

1. Wenlock limestone, formerly well known to collectors by the name of the Dudley limestone, which forms a continuous ridge, ranging for about 20 miles from S.W. to N.E., about a mile distant from the nearly parallel escarpment of the Aymestry limestone. The prominence of this rock in Shropshire, like that of Aymestry, is due to its solidity, and to the softness of the shales above and below. It is divided into large concretional masses of pure limestone, and abounds in trilobites, among which the prevailing species are Phacops caudatus (fig. 422.) and Calymene Blumenbachii, commonly called the Dudley trilobite. The latter is often found coiled up like a wood-louse (see fig. 420.).

Fig. 419.

Fig. 419. Graptolithus Ludensis, Murchison. Lower Ludlow.

Fig. 420.

Calymene Blumenbachii, Brong. Wenlock, L. Ludlow, and Aym. limest.

Fig. 421.

LeptÆna depressa. Wenlock.

Fig. 422.

Phacops caudatus, Brong. Wenlock, Aym. limest., and L. Ludlow.

LeptÆna depressa, Sow., is common in this rock, but also ranges through the Lower Ludlow, Wenlock shale, and Caradoc Sandstone.

Fig. 423.

Catenipora escharoides.

Among the corals in which this formation is very rich, the Catenipora escharoides, Lam. (fig. 423.), or chain coral, may be pointed out as one very easily recognized, and widely spread in Europe, ranging through all parts of the Silurian group, from the Aymestry limestone to the bottom of the series.

Another coral, the Porites pyriformis, is also met with in profusion; a species common to the Devonian rocks.

Cystiphyllum Siluriense (fig. 425.) is a species peculiar to the Wenlock limestone. This new genus, the name of which is derived from ??st??, a bladder, and f?????, a leaf, was instituted by Mr. Lonsdale for corals of the Silurian and Devonian groups. It is composed of small bladder-like cells (see fig. 425. b.).

2. The Wenlock Shale, which exceeds 700 feet in thickness, contains many species of brachiopoda, such as a small variety of the Lingula Lewisii (fig. 412.), and the Atrypa reticularis (fig. 414.) before mentioned, and it will be seen that several other fossils before enumerated range into this shale.

Fig. 424.

Porites pyriformis, Ehren. Wenlock limest. and shale. Also in Aymestry limestone, and L. Ludlow.

a. Vertical section, showing transverse lamellÆ.

LOWER SILURIAN ROCKS.

The Lower Silurian rocks have been subdivided into two portions.

1. The Caradoc sandstone, which abuts against the trappean chain called the Caradoc Hills, in Shropshire. Its thickness is estimated at 2500 feet, and the larger proportion of its fossils are specifically distinct from those of the Upper Silurian rocks. Among them we find many trilobites and shells of the genera Orthoceras, Nautilus, and Bellerophon; and among the Brachiopoda the Pentamerus oblongus and P. lÆvis (fig. 426.), which are very abundant and peculiar to this bed; also Orthis grandis (fig. 427.), and a fossil of well-defined form, Tentaculites annulatus, Schlot. (fig. 428.), which Mr. Salter has shown to be referable to the Annelids and to the same tribe as Serpula.

The most ancient bony remains of fish yet discovered in Great Britain are those obtained from the Wenlock limestones; but coprolites referred to fish occur still lower in the Silurian series in Wales.

2. The Llandeilo flags, so named from a town in Caermarthenshire, form the base of the Silurian system, consisting of dark-coloured micaceous grit, frequently calcareous, and distinguished by containing the large trilobites Asaphus Buchii and A. tyrannus, Murch., both of which are peculiar to these rocks. Several species of Graptolites (fig. 430.) occur in these beds.

In the fine shales of this formation Graptolites are very abundant. I collected these same bodies in great numbers in Sweden and Norway in 1835-6, both in the higher and lower shales of the Silurian system; and was informed by Dr. Beck of Copenhagen, that they were fossil zoophytes related to the genera Pennatula and Virgularia, of which the living species now inhabit mud and slimy sediment. The most eminent naturalists still hold to this opinion.

A species of Lingula is met with in the lowest part of the Llandeilo beds; and it is remarkable that this brachiopod is among the earliest, if not the most ancient animal form detected in the lowest Silurian of North America. These inhabitants of the seas, of so remote an epoch, belonged so strictly to the living genus Lingula, as to demonstrate, like the pteriform ferns of the coal, through what incalculable periods of time the same plan and type of organization has sometimes prevailed.

Among the forms of trilobite extremely characteristic of the Lower Silurian throughout Europe and North America, the Trinucleus may be mentioned. This family of crustaceans appears to have swarmed in the Silurian seas, just as crabs, shrimps, and other genera of crustaceans abound in our own. Burmeister, in his work on the organization of trilobites, supposes them to have swum 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. They underwent various transformations analogous to those of living crustaceans. M. Barrande, author of a work on the Silurian rocks of Bohemia, has traced the same species from the young state just after its escape from the egg to the adult form, through various metamorphoses, each having the appearance of a distinct species. Yet, notwithstanding the numerous species of preceding naturalists which he has thus succeeded in uniting into one, he announces a forthcoming work in which descriptions and figures of 250 species of Trilobite will be given.

CystideÆ.—Among the additions which recent research has made to the paleontology of the oldest Silurian rocks, none are more remarkable than the radiated animals called CystideÆ. Their structure and relations were first elucidated in an essay published by Von Buch at Berlin in 1845. They are usually met with as spheroidal bodies covered with polygonal plates, with a mouth on the upper side, and a point of attachment for a stem b (which is almost always broken off) on the lower. (See fig. 433.) They are considered by Professor E. Forbes as intermediate between the crinoids and echinoderms. The SphÆronites here represented (fig. 433.) occurs in the Llandeilo beds in Wales.[358-A]

Thickness and unconformability of Silurian strata.—According to the observation of our government surveyors in North Wales, the Lower Silurian strata of that region attain, in conjunction with the contemporaneous volcanic rocks, the extraordinary thickness of 27,000 feet. One of the groups, called the trappean, consisting of slates and associated volcanic ash and greenstone, is 15,000 feet thick. Another series, called the Bala group, composed of slates and grits with an impure limestone rich in organic remains, is 9,000 feet thick.[359-A]

Throughout North Wales the Wenlock shales rest unconformably upon the Caradoc sandstones; and the Caradoc is in its turn unconformable to the Llandeilo beds, showing a considerable interval of time between the deposition of this group and that of the formations next above and below it. The Caradoc sandstone in the neighbourhood of the Longmynd Hills in Shropshire, appears to Professor E. Forbes to have been a deep-sea deposit formed around the margin of high and steep land. That land consisted partly of upraised Llandeilo flags and partly of rocks of still older date.[359-B]

Such evidence of the successive disturbance of strata during the Silurian period in Great Britain is what we might look for when we have discovered the signs of so grand a series of volcanic eruptions as the contemporaneous greenstones and tuffs of the Welsh mountains afford.

Silurian Strata of the United States.

The position of some of these strata, where they are bent and highly inclined in the Appalachian chain, or where they are nearly horizontal to the west of that chain, is shown in the section, fig. 379. p. 327. But these formations can be studied still more advantageously north of the same line of section, in the states of New York, Ohio, and other regions north and south of the great Canadian lakes. Here they are found, as in Russia, in horizontal position, and are more rich in well-preserved fossils than in almost any spot in Europe. The American strata may readily be divided into Upper and Lower Silurian, corresponding in age and fossils to the European divisions bearing the same names. The subordinate members of the New York series, founded on lithological and geographical considerations, are most useful in the United States, but even there are only of local importance. Some few of them, however, tally very exactly with English divisions, as for example the limestone, over which the Niagara is precipitated at the great cataract, which, with its underlying shales, agrees paleontologically with the Wenlock limestone and shale of Siluria. There is also a marked general correspondence in the succession of fossil forms, and even species, as we trace the organic remains downwards from the highest to the lowest beds.

Mr. D. Sharpe, in his report on the mollusca collected by me from these strata in North America[359-C], 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 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 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.

Whether the Silurian rocks are of deep-water origin.—The grounds relied upon by Professor E. Forbes, for inferring that the larger part of the Silurian Fauna is indicative of a sea more than 70 fathoms deep, are the following: first, the small size of the greater number of conchifera; secondly, the paucity of pectinibranchiata (or spiral univalves); thirdly, the great number of floaters, such as Bellerophon, Orthoceras, &c.; fourthly, the abundance of orthidiform brachiopoda; fifthly, the absence or great rarity of fossil fish.

It is doubtless true that some living TerebratulÆ, on the coast of Australia, inhabit shallow water; but all the known species, allied in form to the extinct Orthis, inhabit the depths of the sea. It should also be remarked that Mr. Forbes, in advocating these views, was well aware of the existence of shores, bounding the Silurian sea in Shropshire, and of the occurrence of littoral species of this early date in the northern hemisphere. Such facts are not inconsistent with his theory; for he has shown, in another work, how, on the coast of Lycia, deep-sea strata are at present forming in the Mediterranean, in the vicinity of high and steep land.

Had we discovered the ancient delta of some large Silurian river, we should doubtless have known more of the shallow, and brackish water, and fluviatile animals, and of the terrestrial flora of the period under consideration. To assume that there were no such deltas in the Silurian world, would be almost as gratuitous an hypothesis, as for the inhabitants of the coral islands of the Pacific to indulge in a similar generalization respecting the actual condition of the globe.[360-A]

Mineral Character of Silurian Strata.

In lithological character, the Silurian strata vary greatly when we trace them through Europe and North America. The shales called mudstones are as little altered from some deposits, found in recent submarine banks, as are those of many tertiary formations. We meet with red sandstone and red marl, with gypsum and salt, of Upper Silurian date, in the Niagara district, which might be mistaken for trias. The whitish granular sandstone at the base of the Silurian series in Sweden resembles the tertiary siliceous grit of Fontainebleau. The Calcareous Grit, oolite, and pisolite of Upper Silurian age in Gothland, are described by Sir R. Murchison as singularly like rocks of the oolitic period near Cheltenham; and, not to cite more examples, the Wenlock or Dudley limestone often resembles a modern coral-reef. If, therefore, uniformity of aspect has been thought characteristic of rocks of this age, the idea must have arisen from the similarity of feature acquired by strata subject to metamorphic action. This influence, seeing that the causes of change are always shifting the theatre of their principal development, must be multiplied throughout a wider geographical area by time, and become more general in any given system of rocks in proportion to their antiquity. We are now acquainted with dense groups of Eocene slates in the Alps, which were once mistaken by experienced geologists for Transition or Silurian formations. The error arose from attaching too great importance to mineral character as a test of age, for the tertiary slates in question having acquired that crystalline texture which is in reality most prevalent in the most ancient sedimentary formations.

CAMBRIAN GROUP.

Below the Silurian strata in North Wales, and in the region of the Cumberland lakes, there are some slaty rocks, devoid of organic remains, or in which a few obscure traces only of fossils have been detected (for which the names of Cambrian and Cumbrian have been proposed). Whether these will ever be entitled by the specific distinctness of their fossils to rank as independent groups, we have not yet sufficient data to determine.

TABULAR VIEW OF FOSSILIFEROUS STRATA,

Showing the Order of Superposition or Chronological Succession of the principal European Groups.