chap. yiii. Gravel Formation of Patagonia. 219 



flows, and that farther southward there were other sea- 

 straits, since closed. I may add, that at Santa Cruz, 

 in lat. 50° S., the plains have been uplifted at least 

 1,400 feet, since the period when gigantic boulders 

 were transported between sixty and seventy miles from 

 their parent rock, on floating icebergs. 



Lastly, considering the great upward movements 

 which this long line of coast has undergone, and the 

 proximity of its southern half to the volcanic axis of 

 the Cordillera, it is highly remarkable that in the many 

 fine sections exposed in the Pampean, Patagonian 

 tertiary, and Boulder formations, I nowhere observed 

 the smallest fault or abrupt curvature in the strata. 



Gravel Formation of Patagonia. 



I will here describe in more detail than has been as 

 yet incidentally done, the nature, origin, and extent of 

 the great shingle covering of Patagonia : but I do not 

 mean to affirm that all of this shingle, especially that 

 on the higher plains, belongs to the recent period. A 

 thin bed of sandy earth, with small pebbles of various 

 porphyries and of quartz, covering a low plain on the 

 north side of the Rio Colorado, is the extreme northern 

 limit of this formation. These little pebbles have 

 probably been derived from the denudation of a more 

 regular bed of gravel, capping the old tertiary sand- 

 stone plateau of the Rio Negro. The gravel-bed near 

 the Rio Negro is, on an average, about ten or twelve 

 feet in thickness ; and the pebbles are larger than on 

 the northern side of the Colorado, being from one to 

 two inches in diameter, and composed chiefly of rather 

 dark-tinted porphyries. Amongst them I here first 

 noticed a variety often to be referred to, namely, a 

 peculiar gallstone-yellow siliceous porphyry, frequently, 



