Phenomena in South America. 



627 



the most violent action, although it be on a gigantic scale, there we have the 

 best evidence of an almost infinite series of small movements*. 



I vs'ill enter on only one other consideration connected with this subject. 

 From having in my mind the proportional thickness of the strata^, usually given 

 in sections in geological works, I felt much surprise, when I crossed the 

 Cordillera, and found highly-tilted anticlinal lines succeeding one another at 

 short distances, that the rock composing the axis was not to be met with, ex- 

 cept in patches in the valleys. If we suppose parts of the strata in Diagram 

 2 to be placed vertically, the rock of the axis would necessarily be exposed 

 in wide spaces ; but here, I believe, is the source of error, — geologists have 

 not always sufficiently considered the thickness of the mass upturned, in re- 

 lation to the distance of the parallel anticlinal lines one from another. In the 

 Cordillera^ in a width of about sixty miles, there are eight or more anticlinal 

 lines; and thus the centres of the troughs and of the ridges are about four 

 miles apart. Now, if we suppose the upturned crust to be only four miles thick, 

 (which is somewhat more than can actually be seen,) the strata, when placed 

 vertically, will occupy as great an horizontal extension as they did before 

 they were disturbed. In Diagram 3, which I beg it may be understood is 

 given merely to illustrate this one point, I have taken portions of strata of 

 the same exact length as those in Diagram 2 ; but I have increased their 



No. 3. 



thickness, so that it equals the distance of the anticlinal lines from each other; 

 — we shall now see that not only the whole axis is covered, but that the masses 

 cannot be forced into their former horizontal limits. I have not, however, 

 allowed for the immense abrasion which, under such circumstances, the lower 



* Humboldt has insisted on the fact, that in double chains of mountains, such as form large 

 portions of the Andes, the lofty parts of one line correspond with the lower parts of the other. 

 Such symmetry of structure is hardly conceivable on the idea of mountains having been formed 

 by paroxysmal violence ; but if we consider the whole range as the effect of a widely-extended 

 elevation,prolonged during many ages, it is easy to understand, that if one line be weak, and con- 

 sequently be subjected, for a long time, to disturbance from the subterranean force, it is probable 

 that during so much the less time will the parallel and approximate one be affected. 

 VOL. V. SECOND SERIES. 4 M 



