70 PALEOZOIC TIME. [CHAP. VI. 



Nos. 3 and 4 are the Upper Carboniferous series, and are 

 as much as 12,000 feet thick in some parts of Lancashire 

 and Yorkshire. This type prevails over the larger part of 

 England and Ireland. 



The northern type is differently divided in Northumber- 

 land and Scotland, but the following classification would 

 apply to both areas : 



JT ( The Coal-measures (2,000 to 3,000 feet). 



' I The Millstone Grit (100 to 600 feet). 



A limestone group with shales and coals (1,500 



to 3,000). 

 Lower. ^ A carbonaceous shale group (800 to 2,500). 



A sandstone group with basal conglomerates 

 (1,000 to 3,000 feet). 



Certain areas in the north of Ireland exhibit beds of a 

 similar type. 



The records of the Carboniferous period being thus more 

 complete than those of earlier times, and the rocks being 

 more fully exposed and more easily classified, we possess 

 more certain grounds on which to reconstruct the geography 

 of the British area, at any rate during the early stages of 

 the period. 



It is clear that the period was ushered in by the partial 

 submergence of the great continent which included so large 

 a part of Britain in the preceding (Devonian) period. The 

 movement of depression seems to have been very different 

 from the movement which raised that continent ; the up- 

 heaval was effected by a force which acted rather horizon- 

 tally than vertically, forcing up the earth's crust by lateral 

 compression into a series of mighty ridges and furrows. 

 The Carboniferous submergence was apparently an even 

 and uniform downward movement gradually bringing the 

 lower portions of the pre-existent land beneath the level 

 of the sea. 



