A Manual of Elementary Geology Part 54

Ludlow, and Aym. limest.]

[Ill.u.s.tration: Fig. 421. _Leptaena depressa._ Wenlock.]

[Ill.u.s.tration: Fig. 422. _Phacops caudatus_, Brong. Wenlock, Aym.

limest., and L. Ludlow.]

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

[Ill.u.s.tration: 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 +kystis+, a _bladder_, and +phyllon+, a _leaf_, was inst.i.tuted 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.

[Ill.u.s.tration: Fig. 424. _Porites pyriformis_, Ehren. Wenlock limest. and shale. Also in Aymestry limestone, and L. Ludlow.

_a._ Vertical section, showing transverse lamellae.]

[Ill.u.s.tration: Fig. 425. Cystiphyllum.

_a._ _Cystiphyllum Siluriense_, Lonsd. Wenlock.

_b._ Section of portion, showing cells.]

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 sh.e.l.ls of the genera _Orthoceras_, _Nautilus_, and _Bellerophon_; and among the Brachiopoda the _Pentamerus oblongus_ and _P. laevis_ (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_.

[Ill.u.s.tration: Fig. 426. _Pentamerus laevis_, Sow. Caradoc Sandstone.

Perhaps the young of _Pentamerus oblongus_.

_a, b._ Views of the sh.e.l.l itself, from figures in Murchison's Sil. Syst.

_c._ Cast with portion of sh.e.l.l remaining, and with the hollow of the central septum filled with spar.

_d._ Internal cast of a valve, the s.p.a.ce once occupied by the septum being represented by a hollow in which is seen a cast of the chamber within the septum.]

[Ill.u.s.tration: Fig. 427. Cast of _Orthis grandis_, J. Sow. Horderley; two-thirds of nat. size.]

[Ill.u.s.tration: Fig. 428. _Tentaculites scalaris_, Schlot. Eastnor Park; nat. size, and magnified.]

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.

[Ill.u.s.tration: Fig. 429. _Ogygia Buchii_, Burmeister. Syn. _Asaphus Buchii_, Brong. 1/4 nat. size. Radnorshire.]

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.

[Ill.u.s.tration: Fig. 430. _a_, _b_. _Graptolithus Murchisonii_, Beck. Llandeilo flags.]

[Ill.u.s.tration: Fig. 431. _G. foliaceus_, _Murchison_. Llandeilo flags.]

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 a.n.a.logous 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.

[Ill.u.s.tration: Fig. 432. _Trinucleus ornatus_, Burm.]

_Cystideae._--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 _Cystideae_. 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 _Sphaeronites_ here represented (fig. 433.) occurs in the Llandeilo beds in Wales.[358-A]

[Ill.u.s.tration: Fig. 433. _Sphaeronites balticus_, Eichwald. (Of the family _Cystideae_.)

_a._ mouth.

_b._ point of attachment of stem.

Lower Silurian, Shole's Hook and Bala.]

_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 a.s.sociated 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 sh.e.l.ls living near sh.o.r.e. The predominance of bivalve mollusca of this peculiar cla.s.s 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 _Terebratulae_, 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 sh.o.r.es, 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 a.s.sume 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 princ.i.p.al 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.