Chap. VIII.] 
BEITISH SILUKIAN EOCKS. 
185 
in them. It is, however, in the Llandeilo and Caradoc formations that 
Trilobites, Mollnsks, and Corals augment enormously and amount to many- 
hundred species. 
The Llandovery rocks, intermediate between the lower and upper divi- 
sions, swell out in some parts of Wales to a thickness of 2000 or 3000 
feet ; and, though in many tracts this zone is of small dimensions, it is 
not poor in the variety of fossil species. 
The Upper Silurians, consisting of Wenlock and Ludlow rocks, attain 
nowhere a greater thickness than 5000 or 6000 feet ; and yet this younger 
group is as replete with fossils as the Lower Silurian strata, the dimensions 
of which are so much larger. 
In conclusion, it is, indeed, well to reflect upon the fact that, notwith- 
standing their enormous physical development (of not less than 26,000 or 
27,000 feet), the British Silurian rocks are scarcely more copiously charged 
with organic remains than the strata of the same age in Scandinavia, 
where the total united thickness of the Lower and Upper Silurian does not 
exceed 2000 feet, as will be shown in a subsequent chapter. 
