68 INTRODUCTION. 



sea is boundless, and the sedimentary deposits of every ocean 

 must come to an end somewhere. 



An excellent example of the phenomena above described 

 may be derived from the Lower Carboniferous rocks of Bri- 

 tain. Here we may start in South Wales and in Central 

 England with the Carboniferous Limestone as a great calcare- 

 ous 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 sand- 

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

 we have followed the Carboniferous Limestone into York- 

 shire 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, however, still bearing a consi- 

 derable proportion to the whole. Passing still further north- 

 wards, the limestones go on thinning 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 work- 

 able beds of coal, and with but few and comparatively insig- 

 nificant 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 of Scotland, 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 intermixture of mechanical deposits. 



The palaeontological results of this are readily deducible. 

 The entire Lower Carboniferous series of Britain was prob- 

 ably deposited in a single ocean, apparently destitute of land- 

 barriers ; and consequently, taken as a whole, the fauna of 

 this series may be regarded as one and indivisible. The con- 

 ditions, nevertheless, which obtained in different parts of this 



