igoS ] 



THE PHYSICS OF THE EARTH. 171 



Throughout the length of the continent the mountains are every- 

 where parallel to the coast, and run at nearly a constant distance 

 from the shore. The Andes are not always a simple chain, but they 

 are narrow relatively to their height, as compared to the other 

 mountains. In many places there are two or more ranges with 

 narrow plateaus between. These plateaus are so interwoven with 

 the mountains themselves that we may feel sure they were formed 

 together and represent a part of one general movement. Unless 

 this were so it is impossible to believe that so many narrow and high 

 plateaus would be enclosed between mountain walls on either side. 

 The eastern cordillera is less volcanic than the western, and the 

 eastern slope is believed by Professor Solon I. Bailey of Harvard 

 College Observatory, who has exceptional opportunities for judging 

 of these mountains, to be two or three times steeper than the western 

 slope. 



If we suppose a sea trough was first dug out in the elevation of 

 the eastern range, and eventually when deep sediments had ac- 

 cumulated in the trough, the western edge of it was folded up to 

 form the western range, and the trough itself became the plateaus, 

 we shall have very nearly a true picture of how the Andes were 

 formed. The full details of this process cannot be given now, but 

 there is no doubt that the Andes are a vast wall erected by the 

 Pacific along the edge of the continent. This origin of these 

 mountains is also indicated by the earthcjuakes observed within 

 historical time ; for the coast has been again and again upraised 

 by these disturbances, while the sinking of the sea bottom, indicated 

 bv the accompanying seismic sea waves, shows that the bed of the 

 ocean is being undermined by the expulsion of lava under the land. 

 The shells, fossils, and other evidences of marine life now found 

 at altitudes as high as 15,000 feet show that the uplifting at present 

 going on is but a part of the greater uplift of past geological ages; 

 so that the great movement which formed these mountains and pla- 

 teaus is identical with the earthquake disturbances noticed within 

 historical time. 



§ 8. The uplift of mountains and plateaus around the margins 

 of the Pacifie, and of islands in the interior, ztnth innunierable sub- 

 marine eruptions everyzdiere, is nature's zvay of indicating leakage 



