I have just returned from Mendoza, having crossed the Cordilleras by two
passes. This trip has added much to my knowledge of the geology of the
country. Some of the facts, of the truth of which I in my own mind feel
fully convinced, will appear to you quite absurd and incredible. I will
give a very short sketch of the structure of these huge mountains. In the
Portillo pass (the more southern one) travellers have described the
Cordilleras to consist of a double chain of nearly equal altitude separated
by a considerable interval. This is the case; and the same structure
extends to the northward to Uspallata; the little elevation of the eastern
line (here not more than 6,000-7,000 feet.) has caused it almost to be
overlooked. To begin with the western and principal chain, we have, where
the sections are best seen, an enormous mass of a porphyritic conglomerate
resting on granite. This latter rock seems to form the nucleus of the
whole mass, and is seen in the deep lateral valleys, injected amongst,
upheaving, overturning in the most extraordinary manner, the overlying
strata. The stratification in all the mountains is beautifully distinct
and from a variety in the colour can be seen at great distances. I cannot
imagine any part of the world presenting a more extraordinary scene of the
breaking up of the crust of the globe than the very central parts of the
Andes. The upheaval has taken place by a great number of (nearly) N. and
S. lines; which in most cases have formed as many anticlinal and synclinal
ravines; the strata in the highest pinnacles are almost universally
inclined at an angle from 70 deg to 80 deg. I cannot tell you how I
enjoyed some of these views--it is worth coming from England, once to feel
such intense delight; at an elevation from 10 to 12,000 feet there is a
transparency in the air, and a confusion of distances and a sort of
stillness which gives the sensation of being in another world, and when to
this is joined the picture so plainly drawn of the great epochs of
violence, it causes in the mind a most strange assemblage of ideas.