In

phalaspis and Onchus have been discovered.* the Tilestones also, Icthyodorulites, of the genus Onchus, have been obtained; and a species of Dipterus, with mollusca of the genera Avicula, Arca, Cucullæa, Terebratula, Lingula, Turbo, Trochus, Turritella, Bellerophon, Orthoceras, and others.+

By consulting geological maps, the reader will perceive that from Wales to the north of Scotland, the Old Red sandstone appears in patches, and often in large tracts. Many fishes have been found in it at Caithness +, and various organic remains in the northern part of Fifeshire, where it crops out from beneath the Coal formation, and spreads into the adjoining southern half of Forfarshire; forming, together with trap, the Sidlaw hills and valley of Strathmore. (See section, p. 99.) A large belt of this formation skirts the southern borders of the Grampians, from the sea-coast at Stonehaven and the Frith of Tay to the opposite western coast of the Frith of Clyde. In Forfarshire, where, as in Herefordshire, it is many thousand feet thick, it may be divided into three principal masses: 1st, red and mottled marls, cornstone and sandstone; 2d, Conglomerate, often of vast thickness; 3d, Tilestones and paving stone, highly micaceous, and containing a slight admixture of carbonate of

* Murchison's Silurian System, p. 180. + Ibid., p. 183. See Geol. Trans. 2d series, vol. iii. plates 15, 16, 17.

lime. (See section, p. 99.) In the uppermost of these divisions, but chiefly in the lowest, the remains of fish have been found, of the genus named by M. Agassiz, Cephalaspis, or buckler-headed, from the extraordinary shield which covers the head, and which has often been mistaken for that of a trilobite, of the division Asaphus. (See Fig. 276. p. 459.)

Cephlaspis Lyellii, Agass. Length 6 inches.

This figure is from a specimen now in my collection, which I procured at Glammiss, in Forfarshire; see other figures, Agassiz, vol. ii. Tab. 1. a. & 1. b.

a, one of the peculiar scales with which the head is covered when perfect. These scales are generally removed, as in the specimen above figured.

b, c, scales from different parts of the body and tail.

A gigantic species of fish of the genus Gyrolepis has also been found by Dr. Fleming in the Old Red sandstone of Fifeshire.*

455

CHAPTER XXII.

PRIMARY FOSSILIFEROUS STRATA.

Primary Fossiliferous or Transition Strata -Term "Grauwacké” — Silurian Group — Upper Silurian and Fossils Lower Silurian and Fossils - Trilobites

Graptolites Orthocerata - Occasional horizontality of Silurian Strata Cambrian Group - Endosiphonite.

We have now arrived in the descending order at those more ancient sedimentary rocks, which I have called the Primary Fossiliferous (see p. 268.), and to which Werner first gave the name of Transition, for reasons fully explained and discussed in the 12th chapter. Many geologists have also applied to these older strata the general name of "grauwacké," by which the German miners designate a variety of quartzose sandstone, which is usually an aggregate of small fragments of quartz, flinty-slate (or Lydian stone), and clayslate, cemented together by argillaceous matter. But far too much importance has been attached to this kind of rock, as if it were peculiar to a certain epoch in the earth's history, whereas a similar sandstone or grit is not only found sometimes in the Old Red, and in the millstone grit of the Coal, and in certain cretaceous formations of the Alps-but even in some tertiary deposits.

In England, the Old Red sandstone has been

generally regarded as the base of the secondary series; but by some writers on the Continent, the Old Red and Coal formations have been classed as the upper members of the Transition series, a method adopted by Dr. Buckland, in his late Bridgewater Treatise. This classification, however, requires us to draw a strong line of demarcation between the Coal and the lower New Red sandstone group, which now that the fossils of these two groups are ascertained to be very analogous, becomes a more arbitrary division than that which separates the Old Red from the uppermost of the primary fossiliferous strata.

Professor Sedgwick and Mr. Murchison have lately proposed to subdivide all the English sedimentary strata below the Old Red sandstone into two leading groups, the upper of which may be termed the Silurian, and the inferior the Cambrian system. Mr. Murchison has applied the name of Silurian to the newer group, because these rocks may be best studied in that part of England and Wales which was included in the ancient British kingdom of the Silures. He has also formed four subdivisions of the Silurian system, which he has designated as the Ludlow, Wenlock, Caradoc, and Llandeilo, indicating thereby the places where the prevailing characters of each formation are most perfectly exhibited. The following Table explains the succession of these deposits.*

* See Murchison's Silurian System.