LIFE ON THE EARTH. 53 



district appear in the upper part of one series, and 

 in another district in the lower part of another. 

 Thus in several parts of England the Permian system 

 is unconformed to the Coal series, but on the borders 

 of the Staffordshire Coal-field it seems otherwise, and 

 the sequence appears to be nearly complete. Again, 

 the passage of the Permian strata into the Triassic 

 group is more full in the south of Yorkshire than 

 elsewhere, and the passage of the Trias into Lias is 

 best exhibited (in England) in the Vale of the 

 Severn. The uppermost chalk of Maestricht and 

 the region of Bayonne softens the break which in 

 England appears so decisive at the base of the Cseno- 

 zoic strata ; and thus, with few exceptions, by taking 

 our data from different regions we acquire nearly but 

 not quite a complete series of the strata of marine 

 origin, and the means of accurately placing the monu- 

 ments of the life which has passed away for ever. 



SUCCESSIVE SYSTEMS OF LIFE. 



Passing from the limited consideration of man 

 and his works, and the animals and plants specially 

 associated with him, to examine as a third problem 

 how far into antiquity we can trace the forms of the 

 numerous species of existing plants and animals, we 

 find in deposits of old, but perhaps not prehistoric 



