Ill] CARBONIFEROUS ROCKS. 41 



sediments. These were eventually succeeded by less pure 

 calcareous deposits, the sea became shallower, and land detritus 

 found its way over an area formerly occupied by the clear 

 waters of an open sea. The shallowing process was gradually 

 continued, and the sea was by some means converted into a 

 more confined fresh-water or brackish area, in which were laid 

 down many hundred feet of coarse sandy sediments derived 

 from the waste of granitic highlands. Finally the conditions 

 became less constant ; the continuous deposition of sandy 

 detritus being interrupted by the more or less complete filling 

 up of the area of sedimentation, and the formation of a land 

 surface which supported a luxuriant vegetation, of which the 

 debris was subsequently converted into beds of coal. By 

 further subsidence the land was again submerged, and the 

 forest-covered area became overspread with sands and muds. 



Such are the imperfect outlines of the general physical 

 conditions which are represented by the series of sedimentary 

 strata included in the Carboniferous system. At the close 

 of this period, the Earth's surface in Western Europe was 

 subjected to crust-foldings on a large scale, along lines running 

 approximately North and South and East and West, the two 

 sets of movements resulting in the formation of ridges of Car- 

 boniferous rocks. The uppermost series of grits, sandstones 

 and coal-seams were in great part removed by denudation from 

 the crests of the elevated ridges, but remained in the inter- 

 vening troughs or basins where they were less exposed to 

 denudation. It is the direct consequence of this, that we have 

 our Coal-Measures preserved in the form of detached basins of 

 upper Carboniferous beds. 



A closer examination of the comparative thickness and 

 succession of Carboniferous rocks in different parts of Britain 

 shows very clearly that in the northern area of Scotland and 

 in the North of England the conditions were different from 

 those which obtained further South. Seeing how much palaeo- 

 botanical interest attaches to these rocks, it is important to 

 treat a little more fully of their geology. 



In parts of Devon, Cornwall and West Somerset, the Devo- 

 nian strata are succeeded by a series of folded and contorted 



