136 Physical Geography. 



fications of shale and sandstone with occasional beds of 

 coal that lie between them, and which, excepting the 

 beds of coal, were of ordinary aqueous sediments. 



This naturally leads to the question under what 

 circumstances were the purely mechanical sediments 

 and the beds of coal formed ? The answer is, that 

 after the close of the Carboniferous Limestone epoch 

 in the south, the area got filled up by the sands of the 

 Millstone Grit and the more muddy strata (now shales 

 and sandstones) that overlie them, and this shallowing 

 of the seas may have been aided by partial upheaval of 

 the area, till part of it was nearly at, and at length a little 

 above, the level of the sea. Through this flat conti- 

 nental land, great rivers ran, bordered by wide marshy 

 flats, on which the vegetation grew that by its decay 

 and death became transformed into peat. Then by 

 gradual depression these areas were again covered with 

 water, in the first instance salt or fresh, as the case 

 might be, but in all cases resulting in the deposition of 

 layers of sediment. The area was thus converted by 

 degrees into low land, covered by vegetation, a new 

 growth and decay took place, and it was again depressed 

 beneath the water to receive newer sediments, and so 

 on through a vast period of time, till, for example, all 

 the 10,000 feet of the South Wales coal-field were 

 accumulated, interstratified with the hundred beds of 

 coal, great and small, that lie among the shales and 

 sandstones ; and in equal or less degree the same was 

 the case with all the other coal-fields of England and 

 Wales, as far north as those of Lancashire and 

 Yorkshire. 



But when we come to other Carboniferous areas, 

 further north, the case is somewhat different. There 

 we find, in Durham, Northumberland, and Scotland, no 



