IMPERFECTION OF PAL^ONTOLOGICAL RECORD. 37 



In a general way this holds good, not only for any particular 

 bed, but for any particular aggregation or group of beds which 

 we may choose to take. In the case, namely, of every group 

 of beds, there must have been a particular point whither sedi- 

 ment was most abundantly conveyed, or where the other con- 

 ditions of accumulation were especially favourable. At this 

 point, therefore, the beds are thickest, and from this they thin 

 out in all directions. It need scarcely be pointed out, indeed, 

 that some such state of things is 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 example of the phenomena above described 

 may be derived from the Lower Carboniferous Rocks of 

 Britain. 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 sandstone, 

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

 followed the Carboniferous Limestone into Yorkshire and West- 

 moreland, 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 lime- 

 stones, however, still bearing a considerable proportion to the 

 whole. Passing still further northwards, the limestones go on 

 thinning out, till in Central Scotland, in place of the dense cal- 

 careous 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 fol- 

 lows : 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 mechani- 

 cal 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 con- 

 ditions for lime-making were most favourable, and here conse- 

 quently 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 probably 

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



