CHAP. VI.] CARBONIFEROUS PERIOD. 91 



and thus formed lenticular beds of sandstone, or washed 

 down and spread out pre-existing sandbanks, and gave rise 

 to more regular sandstone beds. When the depth was 

 greater, only fine mud was brought into the water, which 

 settled down into more regularly bedded and uniform 

 deposits of shale. 



" The subsidence which allowed of the growth of the 

 mechanically-formed deposits did not go on without inter- 

 ruption. Every now and then a pause occurred, and when- 

 ever this happened, the water became filled up, and there 

 was a tendency to the formation of low, swampy flats." 

 He proceeds to point out that wherever a land surface was 

 formed, vegetation quickly sprang up, and furnished the 

 material for beds of coal. When these swampy flats were 

 again submerged, fresh deposits of mud and sand covered 

 the rich, carbonaceous soil, and entombed the trees and 

 plants which grew in it. 



Such, according to Professor Green, is a brief sketch of 

 the submarine changes attendant upon the gradual depo- 

 sition of the Carboniferous rocks of Yorkshire, and it is 

 applicable to all other regions where a similar series of 

 beds is found. As far as the earlier members of this 

 series are concerned, there is nothing very remarkable in 

 their mode of formation. They are simply masses of 

 limestone, shale, and sandstone which can be paralleled by 

 similar groups in other geological systems ; but the upper 

 members were accumulated under conditions which have 

 been much less frequently repeated during geological 

 time conditions, indeed, which have never prevailed again 

 in the European area to the same extent as they did in the 

 Carboniferous period. It will, therefore, be worth while 

 to describe these conditions a little more fully, and to 

 see what conclusions may be drawn from them as to the 

 general state of physical geography towards the close of 

 this long period. 



