Jan. 1834. 



GEOLOGY. 



201 



have been removed^ thus to leave these mere points, as mea- 

 ures of the former table-land. 



I will now give a brief sketch of the geology of the grand 

 tertiary formation of Patagonia, which extends from the 

 Strait of Magellan to the Bay of S. Antonio. In Europe, 

 deposits of the more recent eras have generally been accumu- 

 lated in small basins or trough-shaped hollows. In South 

 America, however, the entire plains of Patagonia extending 

 seven hundred miles in length, and backed on the one hand 

 by the chain of the Andes, and fronted on the other by the 

 shores of the Atlantic, are thus constituted. Moreover the 

 northern boundary is merely assumed in consequence of a 

 mineralogical change in the strata : if organic remains were 

 present, it probably would be found to be only an artificial 

 limit. Again to the northward (1300 miles distant from the 

 Strait of Magellan) we have the Pampas deposit, Avhich 

 though very different in composition, belongs to the same 

 epoch with the superficial covering of the plains of Pata- 

 gonia. 



The cliffs on the coast give the following section: The 

 lower part consists of a soft sandstone, containing large con- 

 cretions of a harder nature. These strata contain many 

 organic remains — immense oysters nearly a foot in diameter, 

 curious pectens, echini, turritellse, and other shells, of which 

 the greater portion are extinct, but a few resemble those now 

 existing on the coast.* Above these fossiliferous beds, a 

 mass of soft friable stone or earth is superimposed, which, 

 from its extreme whiteness, has been mistaken for chalk. It is, 

 however, quite different ; and closely resembles the less argil- 

 laceous varieties of decomposed felspar. This substance 

 never contains organic remains. Lastly, the cliff is sur- 

 mounted by a thick bed of gravel, almost exclusively 

 derived from porphyritic rocks. For the sake of making 

 the following description more easily intelligible, I have 

 subjoined an imaginary section of the plains near the coast. 



* The geologist must recollect this is a mere sketch, and that the fossil 

 shells have not yet been carefully examined. 



