444 Darwin 9 s Letters to Professor Hensloiv. 



which the solid rock is shivered. In the town there is not one house ha- 

 bitable : the ruins remind me of the drawings of the desolated eastern cities. 

 We were at Valdivia at the time, and felt the shock very severely. The 

 sensation was like that of skating over very thin ice ; that is, distinct un- 

 dulations were perceptible. The whole scene of Concepcion and Talcuana 

 is one of the most interesting spectacles we have beheld since leaving 

 England." 



Elevation of the Land. — " The whole of the coast from Chiloe to the 

 south extreme of the peninsula of Tres Montes is composed of mica-slate. 

 It is traversed by very numerous dykes, the mineralogical nature of which 

 will, I suspect, turn out very curious. I examined one grand transverse 

 chain of granite, which has clearly burst up through the overlying slate. 

 At the peninsula of Tres Montes there has been an old volcanic focus, 

 which corresponds to another in the north part of Chiloe. I was much 

 pleased, at Chiloe, by finding a thick bed of recent oyster-shells, &c, capping 

 the tertiary plain, out of which grew large forest trees. I can now prove 

 that both sides of the Andes have risen, in this recent period, to a consider- 

 able height. Here the shells were 350 ft. above the sea." 



Geology of the Cordilleras. — * Valparaiso, April 18. 1835. I have just re- 

 turned from Mendoza, having crossed the Cordilleras by two passes. This 



trip has added much to my knowledge of the geology of the country 



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 ex- 

 tends northward to Uspellata. The little elevation of the eastern line 

 (here not more than 6000ft. or 7000 ft.) has caused it almost to be over- 

 looked. To begin with the western and principal chain, where the sections 

 are best seen ; we have 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. On 

 the bare sides of the mountains, the complicated dykes and wedges of va- 

 riously coloured rocks are seen traversing, in every possible form and shape, 

 the same formations, which, by their intersections, prove a succession of 

 violences. The stratification in all the mountains is beautifully distinct, 

 and, owing to a variety in their colouring, can be seen at great distances. 1 

 cannot imagine any part of the world presenting a more extraordinary 

 scene of the breaking up of the crust of the globe, than these central peaks 

 of the Andes. The upheaval has taken place by a great number of (nearly) 

 north and south lines* ; which, in most cases, has formed as many anticlinal 

 and synclinal ravines. The strata in the highest pinnacles are almost univer- 

 sally inclined at an angle from 70° to 80° The formation which I call 



porphyritic conglomerates is the most important and most developed in Chili. 

 From a great number of sections, 1 find it to be a true coarse conglomerate, 

 or breccia, which passes by every step, in slow gradation, into afine clay-stone 

 porphyry ; the pebbles and cement becoming porphyritic, till at last all is 

 blended in one compact rock. The porphyries are excessively abundant in 

 this chain ; and I feel sure that at least four fifths of them have been thus 

 produced from sedimentary beds in situ. There are also porphyries which 

 have been injected from below amongst the strata, and others ejected, which 

 have flowed in streams ; and I could show specimens of this rock, produced 

 in these three methods, which cannot be distinguished. It is a great mistake 

 to consider the Cordilleras (here) as composed only of rocks which have 

 flowed in streams. In this range I nowhere saw a fragment which I believe 

 to have thus originated, although the road passes at no great distance from 



* Of dykes. 



