PAL^ONTOLOGICAL RECORD. OGk) 



this they thin off in all directions. It need scarcely be pointed out, 

 indeed, that some such state of things is absolutely unavoidable in 

 the case of every bed or group of beds; since no sea is boundless, 

 and the sedimentary deposits of every ocean must come to an end 

 somewhere. 



An excellent illustration of the phenomena above described may 

 be derived from the Lower Carboniferous Rocks of Britain. Here 

 we may start in the south of Wales and in Central England with the 

 Carboniferous Limestone as a great calcareous mass over 1,000 feet 

 thick, without almost a single intercalated layer of shale. Passing 

 northwards, some of the strata of limestone begin to thin out, and 

 their place is taken by beds 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 

 Westmoreland, in place of a single gi'eat mass of limestone we have 

 now an equivalent mass composed of alternating strata of limestone, 

 sandstone, grit and shale, with two or three thin seams of coal, the 

 limestones, however, still bearing a considerable proportion to the 

 whole. The limestones, however, continue to thin out as we pass 

 northwards, till in Central Scotland, in place of the dense calcareous 

 accumulations of Derbyshire, the Lower Carboniferous series consists 

 of a great gi-oup of sandstones, grits, and shales, with thick 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 of Scotland, 

 and the deepest part of the ocean must have been somewhere in the 

 latitude of Derbyshire and South Wales. Here the conditions for 

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

 the greatest thickness of calcareous strata and the smallest intermix- 

 ture of mechanical deposits. 



The palseontological results of this are readily deducible. The 

 entire Lower Carboniferous series of Britain, was 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 indi- 

 visible. The conditions, nevertheless, which obtained in different 



