igoS.] THE PHYSICS OF THE EARTH. 269 



ginia. Heretofore these vast billows of the earth's crust have been 

 utterly bewildering to the naturalist. 



7. When several such trenches have been dug out, and the ex- 

 pulsion of lava is from both sides, as happens when the sea is thus 

 distributed, the ridges may finally be forced up and so crowded 

 together from both sides that overturned dips and inverted strata 

 are produced, as in the Swiss Alps. No previous theory has been 

 adequate to account for this amazing phenomenon, the explanation 

 of which is thus seen to be exceedingly simple. This test may be 

 justly considered the experimentiun crucis of the theories of moun- 

 tain formation. 



8. The Andes in South America are nothing but a vast wall or 

 embankment erected by the Pacific Ocean, through the expulsion 

 of lava, along its border. Hence the persistence of the earthquake 

 belt and seismic sea waves along this coast. 



9. This embankment includes not only the peaks and chains of 

 mountains, large and small, in the Eastern and Western Cordillera, 

 but also the intervening plateaus, such as those of Quito, Caxa- 

 marca, Cuzco, and Titicaca. 



10. The molten rock expelled from under the sea is lighter than 

 average material of the layer below the earth's crust, and when the 

 included vapor of steam is allowed to expand, as in volcanoes, 

 pumice is formed, and often blow out in vast quantities. Pumice 

 of various degrees of density underlies the mountain chains, and 

 some of it is blown out of those mountains which become volcanoes. 



11. The way in which these plateaus are interwoven with the 

 Andes mountains shows that the whole embankment is due to the 

 continued action of one common cause. And since the mountains 

 were uplifted by the expulsion of lava from under the sea, as proved 

 by the uplifting of the land in earthquakes and the sinking of the 

 sea bottom, indicated by the accompanying seismic sea waves, it 

 follows that the plateaus also are underlaid by matter lighter than 

 the average, which has been expelled from under the ocean. 



12. The total quantity of matter thus expelled from beneath the 

 ocean is very large, but it is the result of an infinite number of 

 earthquakes and seismic sea waves during past geological ages. 

 This circumstance affords us an idea of the immense age of the 



PROC. AMER. PHIL. SOC. , XLVU. 189 R, PRINTED SEPTEMBER 25, I908. 



