Part 53 (1/2)
[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.