A Manual of Elementary Geology Part 53

[350-A] Memoir on the Hartz, Palaeontographica of Dunker and Von Meyer, part iii.

CHAPTER XXVII.

SILURIAN GROUP.

Silurian strata formerly called transition--Term grauwacke--Subdivisions of Upper and Lower Silurian--Ludlow formation and fossils--Wenlock formation, corals and sh.e.l.ls--Caradoc and Llandeilo beds--Graptolites--Lingula--Trilobites--Cystideae--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 "grauwacke," 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 const.i.tuted 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 Thickness Organic Lithological in Feet. Remains.

characters.

{ {Finely laminated } } {Tilestones. { reddish and }800? } { { green sandstones } } { { and shales. } } 1. Ludlow { }Marine mollusca of formation {Upper {Micaceous grey } } almost every order, {Ludlow. { sandstone. } } the Brachiopoda most { } } abundant. Serpula, {Aymestry {Argillaceous } } Corals, Sauroid fish, {limestone. { limestone. }2000 } Fuci.

{ } } {Lower {Shale, with } } {Ludlow. { concretions of } } { { limestone. } }

{Wenlock }Concretionary } {Marine mollusca of {limestone. } limestone. } { various orders as 2. Wenlock { } }1800 { before, Crustaceans formation. { } } { of the Trilobite { } } { family.

{Wenlock }Argillaceous } {Oldest bones of {shale. } shale. } { fish yet known.

LOWER SILURIAN ROCKS.

{Flags of sh.e.l.ly } { { limestone and } {Crinoidea, Corals, 3. Caradoc {Caradoc { sandstone, thick }2500 { Mollusca, chiefly formation. {sandstones. { bedded white } { Brachiopoda, { freestone. } { Trilobites.

4. Llandeilo {Llandeilo }Dark coloured }1200 {Mollusca, formation. {flags. } calcareous flags. } { 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 Shrops.h.i.+re and Herefords.h.i.+re, by peculiar organic remains.

1. _Tilestones._--This uppermost division was originally cla.s.sed 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 pa.s.sage 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 sh.e.l.ls, the _Lingula cornea_, which is common to it and the lowest, or tilestone beds of the Old Red. But the _Orthis...o...b..cularis_ 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.

[Ill.u.s.tration: Fig. 409. _Orthis...o...b..cularis_, J. Sow. Delbury.

Upper Ludlow.]

[Ill.u.s.tration: Fig. 410. _Terebratula navicula_, J. Sow. Aymestry limestone; also in Upper and Lower Ludlow.]

Among the fossil sh.e.l.ls are species of _Leptaena_, _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.

[Ill.u.s.tration: 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 +pente+, _pente_, five, and +meros+, _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 sh.e.l.l, 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.

[Ill.u.s.tration: Fig. 412. _Lingula Lewisii_, J. Sow. Abberley Hills.]

Three other abundant sh.e.l.ls 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.

[Ill.u.s.tration: Fig. 413. _Terebratula Wilsoni_, Sow. Aymestry.]

[Ill.u.s.tration: 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 sh.e.l.ls, 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_.

[Ill.u.s.tration: Fig. 415. _Phragmoceras ventricosum_, J. Sow. (_Orthoceras ventricosum_, Stein.) Aymestry; 1/4 nat. size.]

[Ill.u.s.tration: Fig. 416. _Lituites giganteus_, J. Sow. Near Ludlow; also in the Aymestry and Wenlock limestones; 1/4 nat. size.]

[Ill.u.s.tration: Fig. 417. Fragments of Orthoceras.

_a._ Fragment of _Orthoceras Ludense_, J. Sow.

_b._ Polished section, showing siphuncle. Ludlow.]

The _Orthoceras Ludense_ (fig. 417.), as well as the sh.e.l.l 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 tripart.i.te character of the dorsal crust is almost lost.

[Ill.u.s.tration: Fig. 418. _Homalonotus delphinocephalus_, Konig.[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 Shrops.h.i.+re, 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 ma.s.ses 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.).

[Ill.u.s.tration: Fig. 419. _Graptolithus Ludensis_, Murchison. Lower Ludlow.]

[Ill.u.s.tration: Fig. 420. _Calymene Blumenbachii_, Brong. Wenlock, L.