Page:A Treatise on Geology, volume 1.djvu/148

 132 rocks of different nature, appear to indicate that only at a few exceptional points were the conditions established which allowed of organic life being developed.

No clear or general differences of form distinguish the fossils of the lowest palaeozoic rocks from those of the Silurian system next above: the same predominance of brachiopoda among the shells, the same comparative abundance of zoophyta, and the same rarity of plants, appear to show that the circumstances affecting organic life differed only by degrees. We may, perhaps, consistently view the organic beings of the clay slate and Silurian periods, as belonging to one long succession of creative energy,—the first, if our views as to the origin of the gneiss and mica schist be correct, which was established upon the globe.

Geographical Extent.—In Great Britain, fortunately, this valuable system of rocks is extensively developed along the flanks of the great mountain ranges. A narrow band of clay slate and grauwacke accompanies the south-eastern flank of the Grampians, from Stonehaven, by Dunkeld, Comrie, Loch Venacher, Luss on Loch Lomond, the lower end of Loch Long, part of Bute, and the north-eastern part of Arran. Detached portions occur in Aberdeenshire, at Balachulish, about Dalmally, &c.

As already observed, the greater part of the mountainous region which incloses the Cumbrian Lakes is formed of these rocks, and the slate exists there in great variety of colour, composition, and quality. The slaty cleavage often crosses the interposed trappean masses, as well as the coarsely aggregated argillaceous beds.

In Wales the horizontal area which they cover is greatest on the western side of Snowdonia—about Harlech and Barmouth, and at points in the S. W. extension of the principality toward Prescelly and St. David's.

The Isle of Man is principally composed of slaty rocks, amongst which, perhaps, these oldest rocks have a place. The slate of Charnwood Forest is perhaps (we think probably) to be regarded as of the same period as