Mr, Darwin on certain Volcanic Phcenomena, 589 



The author afterwards discussed the nature and phaBUomena of 

 mountain chains ; and stated his belief, that the injectipn, when in 

 a fluid state, of the great mass of crystalline matter, of which the 

 axis is generally composed, would relieve the subterranean pressure 

 in the same manner as an ejection of lava or scoria ; and that the 

 dislocation of the strata would produce horizontal vibrations through 

 the surrounding country. In drawing this parallel, he also stated his 

 belief, that the earthquake of Concepcion marked one step in the 

 elevation of a mountain chain ; and he adduced, in support of this 

 opinion, the fact observed by Capt. Fitzroy, that the island of Santa 

 Maria, situated 35 miles to the south-west of that city, was elevated 

 to three times the height of the upraised coast near Concepcion ; 

 or at the southern extremity of the island, eight feet ; in the middle, 

 nine feet ; and at the northern extremity, upwards of ten feet ; and 

 that at Tubal, to the south-east of Santa Maria, the land was raised 

 six feet* ; this unequal change of level indicating, in his opinion, 

 an axis of elevation in the bottom of the sea, off the northern end 

 of Santa Maria. 



Mr. Darwin then alluded to Mr. Hopkins's Researches in Physical 

 Geology ,t where it is demonstrated, that if an elongated area were 

 elevated uniformly, it would crack or yield parallel to its longer 

 axis ; and that if the force acted unequally, transverse cracks or fis- 

 sures would be produced, and that the masses, thus unequally dis- 

 turbed, would represent the irregular outline of a mountain- chain. 

 He further added, that if the force should act unequally beneath the 

 area simultaneously affected, various fissures would be formed in 

 different parts, having different directions, and thus give rise, at the 

 same moment, to as many local earthquakes. The author believes, 

 that this view will more readily explain intermediate districts being 

 little disturbed (as Valdivia in 1835, and in cases alluded to by Hum- 

 boldt,) than the supposed inertness of intermediary rock in con- 

 veying the vibrations from a deeply- seated focus. 



If the preceding theory of the cause of earthquakes be true, Mr. 

 Darwin said, we might expect to find, that the many parallel ridges 

 of which the Cordillera is composed, were of successive ages. In 

 Central Chili, the only portion examined by him, this is the case, even 

 with regard to the two main ridges ; and some of the exterior lines 

 of mountains appear, likewise, to be of subsequent dates to the cen- 

 tral ones. The contemplation of these phsenomena led him, while in 

 South America, to infer, that mountain- chains are only subsidiary, 

 and attendant operations on continental elevations. 



The conclusion, that mountain- chains are formed by a long suc- 

 cession of small movements, the author conceived may be arrived at 

 by theoretical reasoning. The first effect of disturbing agents, Mr. 

 Hopkins has shown, is to arch the crust of the earth, and to traverse 



the phsenomena above described and the conchisions above drawn by Mr. 

 Darwin. — Edit.] 



♦ Journal of the Royal Geographical Societj^, vol. vi. p. 327. 



t [See Lend, and Edinb. Phil. Mag. vol. viii. p. 227, et seg.'] 



