igoS.] THE PHYSICS OF THE EARTH. 223 



is much greater than in regions less wrinkled and less afflicted 

 by earthquakes. The crust in the Andes was once folded by the 

 sea in the same way as that in the Aleutian islands, and from this 

 circumstance arises the violence of the volcanic outbreaks noticed 

 all along the west coast of South America. From the great simi- 

 larity of the volcanic phenomena in the Andes and in the Aleutian 

 Islands, and its enormous prominence in both ranges, it seems 

 obvious that we have here the conditions for its maximum devel- 

 opment. 



Charles Darwin believed that volcanoes usually break out in 

 regions of elevation. Xo doubt this is true, for mountain ranges 

 are the most conspicuous of rising areas. And according to this 

 theory the tendency to rupture the crust is a maximum, when the 

 ranges are being both folded and raised from the sea. Thus while 

 some volcanoes may break out in less fractured regions of the earth's 

 crust, the greatest volcanic activity develops where mountains are 

 being formed in the sea. as in the Aleutian Islands. This view also 

 enables us to understand why many volcanoes in the Andes are now 

 extinct, though they were formerly active for immense periods of 

 time, as we knov.' from the thick deposits of volcanic debris and the 

 immense height of the cones built up of lava, ashes and cinders. 



IV. Comparison of the Xew Physical Theory of ^Mountain 

 FoRMATiox Depending on the Leakage of the Oceans 

 WITH THE Theory of Secular Cooling and Con- 

 traction Heretofore Held p.y Men of Science. 



§ 33. General Remarks on the MefJiod of Comparison Adopted. — 

 The new physical theor}- of mountain formation depending on 

 the leakage of the oceans outlined in the three memoirs recently 

 published by the American Philosophical Society and somewhat 

 more fully developed in the present paper might seem incomplete if 

 we failed to compare the new theory with the theory of secular 

 cooling and contraction of the globe heretofore held by men of 

 science generally. On several grounds an examination of the older 

 theory can hardly fail to be instructive. And if this comparison of 

 the older theory with that now adopted shall be the means of bar- 



