§ 57. TERTIARY STRATA OF THE ANDES. 283 



" You will have heard of the dreadful earthquake of the 20th 

 February. I wish some of the geologists, who think the earthquakes 

 of these times are trifling, could see how the solid rocks are shivered. 

 In the town there is not one house habitable ; 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 undulations were 

 perceptible. The whole scene of Conception and Talesana is one of the 

 most interesting spectacles we have beheld since we left England. 

 I was much pleased at Chiloe by finding a thick bed of recent oyster- 

 shells capping the tertiary plain, out of which grew large forest trees. 

 I can prove that both sides of the Andes have risen in this recent 

 period to a considerable height. Here the shells were 350 feet above 

 the sea. On the bare sides of the Cordilleras, complicated dykes and 

 wedges of variously coloured rocks are seen traversing, in every possi- 

 ble form and shape, the same formation, and thus proving by their 

 intersections a succession of violences. The stratification in all the 

 mountains is beautifully distinct, and, owing to a variety of colouring, 

 can be seen at great distances. Porphyritic conglomerates, resting on 

 granite, form the principal masses. I 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 

 strata in the highest pinnacles are almost universally inclined at 

 an angle from 70° to 80°. I cannot tell you how much I enjoyed 

 some of the views : it is alone worth coming from England to feel at 

 once such intense delight. At an elevation of from ten to twelve 

 thousand feet there is a transparency in the air, and a confusion of 

 distances, and a stillness, which give the sensation of being in another 

 world. The most important and most developed formation in Chili is 

 the porphyritic concrete. From a great number of sections, I find it 

 to be a true coarse conglomerate or breccia, which passes by every step 

 in slow gradation to a fine clay-stone porphyry ; the pebbles and cement 

 becoming porphyritic, till at last all is blended in one compact rod:. 

 The porphyries are excessively abundant in this chain, and at least 

 four-fifths of them, I am sure, have been thus produced from sedi- 

 mentary beds in situ. The Uspellata range is geologically, although 

 only six or seven thousand feet high, a continuation of the grand 

 eastern chain. It has its nucleus of granite, consisting of beds of 

 various crystalline rocks, (which I have no doubt are subaqueous 

 lavas.) alternating with sandstone, conglomerates, and white aluminous 

 beds, like decomposed felspar, with many other curious varieties of 

 sedimentary deposits, In an escarpment of compact greenish sandstone, 



