chap. in. Santa Cruz. 381 



of the basaltic lava) composing the great boulders on 

 the surface of the plain, and probably composing the 

 neighbouring Cordillera. Five miles higher up the 

 valley, and again thirty miles higher up, 1 (that is twenty 

 miles from the nearest range of the Cordillera) the lower 

 plain included within the upper escarpments is formed, 

 as seen on the banks of the river, of a nearly similar 

 but finer-grained, more earthy, laminated sandstone, 

 alternating with argillaceous beds, and containing nu- 

 merous moderately sized pebbles of the same rocks, and 

 some shells of the great Ostrea Patagonica. As most 

 of these shells had been rolled before being here em- 

 bedded, their presence does not prove that the sand- 

 stone belongs to the great Patagonian tertiary formation, 

 for they might have been redeposited in it when the 

 valley existed as a sea-strait ; but as amongst the 

 pebbles there were none of basalt, although the cliffs on 

 both sides of the valley are composed of this rock, I 

 believe that the sandstone does belong to this formation. 

 At the highest point to which we ascended, twenty 

 miles distant from the nearest slope of the Cordillera, I 

 could see the horizontally zoned white beds, stretching 

 under the black basaltic lava, close up to the mountains ; 

 so that the valley of the S. Cruz gives a fair idea of the 

 constitution of the whole width of Patagonia. 



Basaltic Lava of the 3. Cruz. — This formation is 

 first met with sixty-seven miles from the mouth of the 

 river ; thence it extends uninterruptedly, generally but 

 not exclusively on the northern side of the valley, close 

 up to the Cordillera. The basalt is generally black 

 and fine-grained, but sometimes gray and laminated ; 

 it contains some olivine, and high up the valley much 



1 I found at both places, but nQt in situ, quantities of coniferous 

 and ordinary dicotyledonous silicined wood, which was examined 

 for me by Mr. E. Brown. 



