Jan. 1834. 



GEOLOGY. 



miles above the mouth of the river, the bed of gravel is 212 

 feet thick, whereas, near the coast, it seldom exceeds 25 or 30 

 feet ; the thickness being thus reduced to nearly one-eighth. 



I have already stated that the gravel is separated from the 

 fossiliferous strata by some white beds of a friable substance, 

 singularly resembling chalk, but which cannot be compared, 

 as far as I am aware, with any formation in Europe. With 

 respect to its origin, I may observe that the well-rounded 

 pebbles all consist of various felspathic porphyries ; and that, 

 from their prolonged attrition, during the successive re- 

 modellings of the whole mass, much sediment must have 

 been produced. I have already remarked that the white 

 earthy matter more closely resembles decomposed felspar, 

 than any other substance. If such is its origin, it would 

 always, from its lightness, be carried further to seaward than 

 the pebbles. But as the land was elevated, the beds would 

 be brought nearer the coast-line, and so become covered by 

 the fresh masses of gravel which were travelling outwards. 

 When these white beds were themselves elevated, they 

 would hold a position intermediate between the gravel and 

 the common foundation, or the fossiliferous strata. To 

 explain my meaning more clearly, let us suppose the bottom 

 of the present sea covered to a certain distance from the 

 coast-line, with pebbles gradually decreasing in size, and 

 beyond it by the white sediment. Let the land rise, so that 

 the beach-line, by the fall of the water, may be carried out- 

 wards; then likewise the gravel, by the same agency as 

 before, will be transported so much further from the coast, 

 and will cover the white sediment, and these beds again will 

 invade the more distant parts of the bottom of the sea. By 

 this outward progress, the order of superposition must 

 always be gravel, white sediment, and the fossiliferous 

 strata. 



Such is the history of the changes by which the present 

 condition of Patagonia has, I believe, been determined. 

 These changes all result from the assumption of a steady 

 but very gradual elevation, extending over a wide area, and 



