330 SEE— THE CAUSE OF EARTHQUAKES. [October 19, 



on the west and the extreme steepness of the descent on the east ;^ 

 and we also account for the more numerous jutting spurs on the 

 west. On the whole side spurs are much less conspicuous on the 

 east, where descent is more rapid. These characteristics are prob- 

 ably the leading features of the Andes, but there are others deserv- 

 ing of attention, among which we may mention the following : 



1. Very great volcanic violence throughout the whole range, 

 and in the peaks of the eastern as well as of the western cordillera. 



2. Enormous vertical uplifts, or fault movements, often amount- 

 ing to thousands of feet, occurring throughout the cordillera, but be- 

 coming especially predominant on the eastern side. 



3. The vertical uplifting of enormous plateaus such as those of 

 Quito, Caxamarca, Cuzco and Titicaca, the latter being 12,500 feet 

 above the sea. 



The heaving of the Andean Valley in the sea seems to be the 

 principal cause of the original elevation of the mountains, but it 

 appears probable that after the mountains were raised to great 

 height another secondary cause contributed to the forces operative 

 in producing the present enormous elevation. This secondary force 

 was nothing else than the soaking tropical rains constantly drench- 

 ing the eastern slope of the mountains. As the earth's crust was 

 already broken and faulted, the leakage of the water downward 

 would be facilitated, while the ceaseless character of the rainfall 

 would make the eastern slope of the Andes to all essential purposes 

 an inland sea. Effectively, therefore, this great range of mountains 

 is built upon a narrow strip of land with seas on both sides like the 

 mountains in Central America, and hence, the violence of the vol- 

 canoes and earthquakes becomes more easily intelligible. 



To make this theory more specific we may recall that all the 

 principal peaks about Quito have been volcanic, and three or four 

 volcanoes are still terribly active there now. Just east of Quito, at 

 the head of the Amazon Valley, are the most terrible rainfalls on 



^ Professor Solon I. Bailey, of Harvard Observatory, who crossed the 

 Andes twice, once near Sorata, Bolivia, and again at the Aricoma Pass, and 

 traversed the eastern rano^e a third time through the river Urnbamba, writes 

 me that his impression is that the eastern slope is tw^o or three times steeper 

 than the western slope. Few observers have had better opportunities of 

 judging of the general structure of the Central x\ndes than Professor Bailey. 



