i9o6.] SEE— THE CAUSE OF EARTHQUAKES. 343 



necessary to add that new ashes, cinders, scoriae, pumice and other 

 volcanic materials are not as a whole red hot, and therefore the bulk 

 of them is not formed at the time of the first volcanic outbreak. 

 They have been resting quietly under the mountain for immeasurable 

 ages, and one cannot doubt that they exist in all mountains. 



It is unfortunate that a custom has arisen of speaking of vol- 

 came action as essentially superficial. While it cannot be said to 

 have any fixed depth, and in many cases may not be extremely deep, 

 it is safe to say that, as a general rule, it is by no means very 

 shallow. On the average the depth is at least of the same order as 

 the height of the mountains, plus the depth of the adjacent sea. 

 There is every reason to suppose the forces which pushed up the 

 Andes and keeps some of them in active eruption arises from about 

 the same depth as the most violent earthquakes which visit that 

 region. Only a few of the peaks of the Andes have become vol- 

 canoes, but some of them, as Aconcagua, in Chili, Gualateiri and 

 Sahama, of the Sorata Range in Bolivia, and Cotopaxi and its 

 associates near Quito in Ecuador ar€ all very high ; and the ejection 

 of materials at that height requires deep-seated and most tremen- 

 dous forces. Yet obviously only a small part of the energy origi- 

 nating under the base of a volcano has been spent in ejecting lava, 

 rocks, ashes and cinders ; a very large part of the energy exerted 

 from below has been spent in raising the sides of the mountain before 

 eruption broke out at the top. 



We may then consider all mountains to be made up internally of 

 light material like that blown out of volcanoes. Chimborazo, for 

 example, may never have had an eruption, and yet it is impossible 

 to believe that its constitution is essentially different from that of 

 the neighboring mountains of Cotopaxi, Pinchinchi, Antisana, 

 Tunguragua, Sangay, and Cayambe, all of which are volcanoes. 



Now it is a very remarkable fact of observation that geodetic 

 researches in many countries, and pretty much ever since the ex- 

 periments of Bouguer and La Condamaine on Chimborazo in the 

 year 1738, have tended to show that the matter under the roots of a 

 great mountain is lighter than the average matter of the adjacent 

 plain. In their experiments on the deviation of the plumb line 

 near Chimborazo the existence of great cavities in this colossal 



PROC. AMER. PHIL, SOC. , XLV. 184 V, PRINTED FEBRUARY 23, I907. 



