THE GROWTH OF BRITAIN 209 



remains must pass towards the land, from which mud is carried out 

 by currents and deposited ; while following in the same direction, 

 we shall pass to the sand and pebbles of the shore, and beyond 

 that again to the land, where denudation and not deposition 

 was taking place. 



Whenever, therefore, we find a limestone passing into a 

 deposit of mechanically denuded matter we are approaching 

 the margin of the sea in which it was deposited, and we may 

 hope, on proceeding farther in the same direction, to reach the 

 shore line, and eventually the land of the period. There is no 

 more striking instance of this than the Carboniferous Limestone, 

 which is pure, thick, and oceanic in Derbyshire. Traced north- 

 wards, beds of clay are associated with it, and become thicker 

 in a northward direction. Then sands come in, then seams of 

 coal, and in Scotland there are even fresh-water limestones and 

 shore deposits on the same horizon. The Carboniferous Lime- 

 stone sea had its main coast-line to the north, and its land was 

 what is now the Highlands of Scotland. Again, tracing the 

 limestone southwards from South Wales and Bristol, where it 

 has much the same character as in Derbyshire, we find it passing 

 into deeper-water beds, such as radiolarian cherts in Devon, 

 and therefore we conclude that this was the deepest part of the 

 British sea in this period. Something similar to this can be 

 worked out for most British Formations, and even the marine 

 Chalk can be traced into shallow-water beds and shore deposits 

 in Scotland. Thus it becomes possible to work out the general 

 outlines of the seas and land areas of past time. 



In this line of inquiry one of the most valuable classes of 

 evidence is derived from the relation of a stratum to those on 

 which it rests. If the surfaces of the older and newer strata are 

 parallel to one another they are said to be conformable (Figs. 18 

 and 19), the sea bed on which deposit was taking place in the 

 second period remaining parallel to its position in the first period, 

 though it may have moved upwards or downwards. But often 

 the dip or strike of a Formation may differ from that which 

 underlies it, and the lowest bed of the second series may rest 

 successively on one member after another of the older series 

 (Fig. 81). This is called unconformity, and it indicates that the 



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