CLASSIFICATION OF THE SILURIAN ROCKS. 23 
of the universal researches of geologists.' It follows, therefore, that the Silurian deposits, 
as a whole, were formed in an incalculably long “ invertebrate”’ period. 
The general succession of life presented to us by fossils is the following :—In the 
oldest sedimentary deposits, Protozoans, Zoophytes, and Annelids only occur; in the next 
epoch, Mollusca, especially Brachiopoda, with Annelida, Crustacea, and Echinodermata. 
These continued to be associated during enormously long periods of time, and were the 
exclusive tenants of the sea. In the following period Fishes appeared, and they were, in 
another era, succeeded by Reptiles ; and then, after another very lengthened time, insecti- 
vorous marsupial Mammals, of small dimensions, appeared. 
In dwelling, however, on the Silurian Rocks only, the paleontologist may next be 
reminded of a few data respecting their geographical extension, physical characters, and 
order,-as they rise to the surface in the British Isles, in a very large portion of the whole 
area of the United Kingdom. Referring, in the first instance, to the original Silurian 
Region proper, or that wherein the system was established, it will be recollected that I 
showed how these rocks occur largely in the following eleven counties, viz., Salop, 
Hereford, Radnor, Montgomery, Brecon, Carmarthen, Pembroke, Monmouth, Gloucester, 
Worcester, and Stafford. In the north-west of England, 7. e., in Lancashire, Cumber- 
land, and Westmoreland, the system has since been seen to be very largely developed, 
and strikingly so in the Lake-country, between Kirby Lonsdale in the south and Skiddaw 
in the north ; it having recently been proved, by their imbedded fossils, that the slates of 
the last-mentioned mountain, or the oldest rocks in Cumberland, are simply equivalent of 
the Lower Llandeilo Rocks of the Silurian Region.? Altogether they occupy an area of 
about 7600 square miles in England and Wales. Then again, in Scotland, with some 
slight exceptions of overlying deposits, Silurian Rocks prevail in the whole of the southern 
counties, from the Solway Firth on the west and from Berwick on the east, to the great 
Carboniferous trough that extends from Fife and Edinburgh to Glasgow and Ayr. 
These are chiefly of Lower Silurian age; but every here and there, as seen in the 
Pentland Hills, near the Scottish metropolis, they exhibit transitions into Middle and 
Upper Silurian, the latter being most completely developed near Lesmahago, in Lanark- 
shire. If to this vast area, throughout which fossils are found at intervals, we add the 
Highland counties, or those north of the Tay and the Clyde, in which rocks of Lower Silurian 
age have undergone great metamorphism, and exhibit fossils at two or three rare localities 
only, it may be stated, that the strata of this great system occupy the surface in Scotland 
to the extent of about 18,420 square miles. | 
In Ireland also, though their exact boundaries have not yet been precisely defined, it 
1 See the works of all the American geologists, and of the Canadian Survey ; Barrande’s ‘ Bassin 
Silurien de Bohéme ;’ ‘ Russia in Europe and the Ural Mountains ;’ Angelin and Swedish authors; and the 
recently published work and map of Kjerulf as respects Norway. 
2 See Harkness’s Memoir, ‘ Quart. Journ. Geol. Soc.,’ vol. xix, page 113, and vol. xxi, p. 235: Mr. H. 
Nicholson discovered most of the fossils therein named. 
