1906.] 



SEE— THE CAUSE OF EARTHQUAKES. 355 



Andes were formed, beginning on the east, and working westward; 

 as the movement progressed the chains got higher and higher, 

 requiring increased volcanic forces ; and in the last range, therefore, 

 occur the most and the highest volcanoes. This principle seems to 

 hold not only in the Andes, but generally throughout the world. 



§ 34. On the formation of islands, and on the significance of the 

 symmetrical disposition of their mountains. 



We have already explained the formation of mountains and have 

 seen that they are not due to any shrinkage of the globe with result- 

 ing collapse of the crust, but to the injection of lava from the sea, 

 which uplifts the crust into a parallel ridge. The subject of island 

 formation, already alluded to in § ;^^, has not been much discussed, 

 and it is impossible to say that there is any recognized theory on the 

 subject. When the islands are small they are justly considered 

 mountains in the sea. Some islands are volcanic and are thus vol- 

 canoes pushed up in the sea like those occasionally raised on the 

 land. That volcanic islands are similar to ordinary land volcanoes 

 seems clear enough ; but how about the larger islands ? How did 

 they originate? To answer this question satisfactorily, we may 

 remark that when we look at an ocean like the Pacific and notice 

 how it is dotted all over with peaks projecting above the water, and 

 remember that a still greater number do not reach the surface, we 

 recognize that such peaks were no part of the original constitution 

 of the globe, but have been developed in the sea in the course of 

 immeasurable ages. 



Now, as to the larger islands, many of them have conspicuous 

 mountain chains, and the study of the lay of these chains is very 

 instructive. If mountains are wrinkles in the earth's crust, due to 

 the shrinkage of the globe, there is certainly no reason why they 

 should be symmetrically placed on islands. On this theory, the 

 mountains might cross the islands at any angle, or even miss the 

 islands entirely. On actual examination, what do we find to be the 

 fact? When we look at any good map we see the mountains run- 

 ning through the islands with the utmost symmetry, making in all 

 cases a veritable backbone or central axis for the land on both sides 

 In no cases do the mountains run diagonally or crosswise. To 

 appreciate the significance of this arrangement, look at the map of 



