IMPERFECTION OF PAL^EONTOLOGICAL RECORD. 73 



derived from the Lower Carboniferous rocks of Britain. Here we 

 may start in South Wales and in Central England with the Carbon- 

 iferous Limestone as a great calcareous mass over 1000 feet in 

 thickness, and almost without a single intercalated layer of shale. 

 Passing northwards, some of the beds of limestone begin to thin out, 

 and their place is taken by strata of a different mineral nature, such 

 as sandstone, grit, or shale. The result of this is, that by the time 

 we have followed the Carboniferous Limestone into Yorkshire and 

 Westmorland, in place of a single great mass of limestone, we have 

 an equivalent mass of alternating strata of limestone, sandstone, grit, 

 and shale, with one or two thin seams of coal — the limestones, how- 

 ever, still bearing a considerable proportion to the whole. Passing 

 still further northwards, the limestones continue to thin out, till in 

 Central Scotland, in place of the dense calcareous accumulations of 

 Derbyshire, the Lower Carboniferous series consists of a great group 

 of sandstones, grits, and shales, with thick and workable beds of 

 coal, and with but few and comparatively insignificant beds of 

 limestone. 



The state of things indicated by these phenomena is as follows : 

 The sea in which the Lower Carboniferous rocks of Britain were 

 deposited must have gradually deepened from north to south. The 

 land and coast-line whence the coarser mechanical sediments were 

 derived, must have been placed somewhere to the north and north- 

 west of what is now Great Britain, and the deepest part of the ocean 

 must have been somewhere about Derbyshire. Here the conditions 

 for lime-making were most favourable, and here consequently we find 

 the greatest thickness of calcareous strata, and the smallest inter- 

 mixture of mechanical deposits. 



The palaeontological results of this are readily deducible. The 

 entire Lower Carboniferous series of Britain was probably deposited 

 in a single ocean, apparently destitute of land-barriers ; and conse- . 

 quently, taken as a whole, the fauna of this series may be regarded 

 as one and indivisible. The conditions, nevertheless, which obtained 

 in different parts of this area were very dissimilar ; and, as a necessary 

 result, certain groups of animals flourished in certain localities, and 

 were absent or but scantily represented in others. In the deeper 

 parts of the area we have an abundance of Corals, with Crinoids, 

 and at times Foraminifera. In the shallower parts of the area there 

 is, on the other hand, a predominance of forms which affect water of 

 no great depth. Still there is no difference in point of time between 

 the deposits of different parts of the area ; and in order to obtain a 

 true notion of the Lower Carboniferous fauna, we must add the 

 fossils derived from one portion of the area to those derived from 

 another. 



In many cases, however, we are acquainted with but one class of 



