1906.] 



SEE— THE CAUSE OF EARTHQUAKES. 



307 



piles up and returns as a great wave which continues to oscillate 

 furiously, sometimes for days after the earthquake. We shall con- 

 sider these waves more fully hereafter, and at present it is sufficient 

 to remark that this explanation is satisfactory for the kind of waves 

 usually observed along the west coast of South America. 



We may then suppose that in such earthquakes a very large 

 mass of lava is forced from under the sea, which then settles below 

 its former level, and the great wave follows. If the lava is forced 

 toward the land, the coast or mountains are upraised ; if towards the 

 ocean, a ridge may be upheaved there, or possibly a submarine vol- 

 cano of large extent. In either case the trough of the sea bottom 

 parallel to the coast eventually becomes less stable, and, at certain 

 intervals, settles little by little, when the consistency of underlying 



Fig. 5. 



^nie3 



lava has been thinned by successive ejections ; and thus with the 

 settling stability is again restored. 



As the trough is arched downwards towards the exploding lava 

 the steam pressure from beneath cannot force it upward; and the 

 strain is necessarily relieved by motion of the lava towards the 

 Andes or the ocean — usually towards the mountains till the trough 

 gets broad and deep and the mountains very far away and so high 

 that the movement of the column offers unprecedentedly great re- 

 sistance, when the release wiM at length become easier towards the 

 ocean by the forcing up of ridges or volcanoes along the other 

 margin of the trough. Ridges with peaks in them will usually re- 

 sult, and this is the beginning of the new Andes or Cordilleras, which 

 are destined to rise slowly from the sea, leaving a deep valley 

 towards the ancient shore, to be drained and filled in by erosion. 



Thus we explain some of the remarkable parallel ridges of 



