RELATIOX OF SEDIMEXTATIOX TO MOUNTAIN RANGES 393 



with marked intrusions of a plntonic type. In the middle Eocene the 

 great thickness of Tertiary sediments was folded. Again, in the Miocene, 

 close folding was revived to the accompaniment of great plntonic intru- 

 sions. The Pliocene marked a plateau uplift of magnificent proportions, 

 to the accompaniment of heavy crushing and folding along the plateau 

 margins. 



A significant fact in connection with existing mountains is that the 

 main plateaus occur in the harder and older rocks, while the basins be- 

 tween them contain the softer and less altered sediments, the latter being 

 crushed on their margins if the bordering ranges are very high, whereas 

 they are gently bowed or warped only if the associated mountains possess 

 but a moderate amount of relief, and that only near the plateau bases. 



Some special Features of Cexozoic Mountains 



Before leaving the subject of the nature of past and existing moun- 

 tains, it may be advisable, perhaps, to consider a few additional points in 

 connection with the Cenozoic types. 



In the first place, they are plateaus with warped margins, and they are 

 the revivals of Eocene and Miocene examples which exhibit certain differ- 

 ences among themselves. 



The earlier mountains may be divided into three groups, all alike, but 

 showing great variations in the quantitative factor. One group is the 

 Tethyan, the members of which have been formed in geosynclines between 

 two dense landmasses during the Eocene and Miocene. The sediment? 

 laid down in these geosynclines were intenseh' folded and were intruded 

 by plutonic rocks. 



The second group is that whose individuals are distributed in and 

 around the Pacific. These show signs of folding during Cenozoic time. 



The third group is the similar, but less important, group bordering the 

 Atlantic and western Indian oceans. 



The inference is that the mountains of Eocene and Miocene time had 

 been comparatively great only in the Tethyan area. Basaltic flows de- 

 vastated the areas of India, western America, eastern Australia, and some 

 other places at the same time. 



The Pliocene and early Pleistocene mountains, on the other hand, were 

 giants whose distribution was cosmopolitan. They exist today, deeply 

 dissected it is true, but with their mighty and original profiles in an 

 excellent state of preservation. 



These mountains of the plateau type were developed not only in the 

 unstable portions of the earth's crust, such as the Tethvan-Pacific mar- 



