Among other Cephalopods in the Llandeilo flags is Cyrtoceras; in the same beds also are found Bellerophon (see [Fig. 488]) and some Pteropod shells (Conularia, Theca, etc.), also in spots where sand abounded, lamellibranchiate bivalves of large size. The Crustaceans were plentifully represented by the Trilobites, which appear to have swarmed in the Silurian seas just as crabs and shrimps do in our own; no less than 263 species have been found in the British Silurian fauna. The genera Asaphus (Fig. 561), Ogygia (Fig. 562), and Trinucleus ([Figs. 552] and 553) form a marked feature of the rich and varied Trilobitic fauna of this age.

Beneath the black slates above described of the Llandeilo formation, Graptolites are still found in great variety and abundance, and the characteristic genera of shells and trilobites of the Lower Silurian rocks are still traceable downward, in Shropshire, Cumberland, and North and South Wales, through a vast depth of shaly beds, in some districts interstratified with trappean formations of contemporaneous origin; these consist of tuffs and lavas, the tuffs being formed of such materials as are ejected from craters and deposited immediately on the bed of the ocean, or washed into it from the land. According to Professor Ramsay, their thickness is about 3300 feet in North Wales, including those of the Lower Llandeilo. The lavas are feldspathic, and of porphyritic structure, and, according to the same authority, of an aggregate thickness of 2500 feet.

Arenig or Stiper-Stones Group (Lower Llandeilo of Murchison).—Next in the descending order are the shales and sandstones in which the quartzose rocks called Stiper-Stones in Shropshire occur. Originally these Stiper-Stones were only known as arenaceous quartzose strata in which no organic remains were conspicuous, except the tubular burrows of annelids (see Fig. 563, Arenicolites linearis), which are remarkably common in the Lowest Silurian in Shropshire, and in the State of New York, in America. They have already been alluded to as occurring by thousands in the Silurian strata unconformably overlying the Cambrian, in the mountain of Queenaig, in Sutherlandshire ([Fig. 82]). I have seen similar burrows now made on the retiring of the tides in the sands of the Bristol Channel, near Minehead, by lob-worms which are dug out by fishermen and used as bait. When the term Silurian was given by Sir R. Murchison, in 1835, to the whole series, he considered the Stiper-Stones as the base of the Silurian system, but no fossil fauna had then been obtained, such as could alone enable the geologist to draw a line between this member of the series and the Llandeilo flags above, or a vast thickness of rock below, which was seen to form the Longmynd hills, and was called ”unfossiliferous graywacke.” Professor Sedgwick had described, in 1843, strata now ascertained to be of the same age as largely developed in the Arenig mountain, in Merionethshire; and the Skiddaw slates in the Lake-District of Cumberland, studied by the same author, were of corresponding date, though the number of fossils was, in both cases, too few for the determination of their true chronological relations. The subsequent researches of Messrs. Sedgwick and Harkness, in Cumberland, and of Sir R. I. Murchison and the Government surveyors in Shropshire, have increased the species to more than sixty. These were examined by Mr. Salter, and shown in the third edition of ”Siluria” (p. 52, 1859) to be quite distinct from the fossils of the overlying Llandeilo flags. Among these the Obolella plumbea, Æglina binodosa, Ogygia Selwynii, and Didymograpsus geminus (Fig. 564), and D. Hirundo, are characteristic.

But, although the species are distinct, the genera are the same as those which characterise the Silurian rocks above, and none of the characteristic primordial or Cambrian forms, presently to be mentioned, are intermixed. The same may be said of a set of beds underlying the Arenig rocks at Ramsay Island and other places in the neighbourhood of St. David’s. These beds, which have only lately become known to us through the labours of Dr. Hicks,[[5]] present already twenty new species, the greater part of them allied generically to the Arenig rocks. This Arenig group may therefore be conveniently regarded as the base of the great Silurian system, a system which, by the thickness of its strata and the changes in animal life of which it contains the record, is more than equal in value to the Devonian, or Carboniferous, or other principal divisions, whether of primary or secondary date.

It would be unsafe to rely on the mere thickness of the strata, considered apart from the great fluctuations in organic life which took place between the era of the Llandeilo and that of the Ludlow formation, especially as the enormous pile of Silurian rocks observed in Great Britain (in Wales more particularly) is derived in great part from igneous action, and is not confined to the ordinary deposition of sediment from rivers or the waste of cliffs.

In volcanic archipelagoes, such as the Canaries, we see the most active of all known causes, aqueous and igneous, simultaneously at work to produce great results in a comparatively moderate lapse of time. The outpouring of repeated streams of lava—the showering down upon land and sea of volcanic ashes—the sweeping seaward of loose sand and cinders, or of rocks ground down to pebbles and sand, by rivers and torrents descending steeply inclined channels—the undermining and eating away of long lines of sea-cliff exposed to the swell of a deep and open ocean—these operations combine to produce a considerable volume of superimposed matter, without there being time for any extensive change of species. Nevertheless, there would seem to be a limit to the thickness of stony masses formed even under such favourable circumstances, for the analogy of tertiary volcanic regions lends no countenance to the notion that sedimentary and igneous rocks 25,000, much less 45,000 feet thick, like those of Wales, could originate while one and the same fauna should continue to people the earth. If, then, we allow that about 25,000 feet of matter may be ascribed to one system, such as the Silurian, as above described, we may be prepared to discover in the next series of subjacent rocks a distinct assemblage of species, or even in great part of genera, of organic remains. Such appears to be the fact, and I shall therefore conclude with the Arenig beds my enumeration of the Silurian formations in Great Britain, and proceed to say something of their foreign equivalents, before treating of rocks older than the Silurian.