202 Valley of Santa Cruz. pakt n. 



and of a Mytilus, which Mr. Sowerby informs me is yet 

 unnamed, though well known as recent on this coast ; 

 Patella deaurita ; Fusus. I believe, Magellanicus, but 

 the specimen has been lost ; and at the distance of four 

 miles from the coast, at the height of about 400 feet, 

 there were fragments of the same Patella and of a 

 Yoluta (apparently V. ancilld) partially embedded in 

 the superficial sandy earth. All these shells had the 

 same ancient appearance with those from the foregoing 

 localities. As the tides along this part of the coast 

 rise at the Syzygal period forty feet, and therefore form 

 a well-marked beach-line, I particularly looked out for 

 ridges in crossing this plain, which, as we have seen, 

 rises 108 feet in about six miles, but I could not see 

 any traces of such. The next highest plain is 710 feet 

 above the sea ; it is very narrow, but level, and is 

 capped with gravel ; it abuts to the foot of the 840 

 feet plain. This summit-plain extends as far as the 

 eye can range, both inland along the southern side ot 

 the valley of the Santa Cruz, and southward along the 

 Atlantic. 



The Valley of the JR. Santa Cruz. — This valley runs 

 in an east and west direction to the Cordillera, a dis- 

 tance of about 160 miles. It cuts through the great 

 Patagonian tertiary formation, including, in the upper 

 half of the valley, immense streams of basaltic lava, 

 which, as well as the softer beds, are capped by gravel ; 

 and this gravel, high up the river, is associated with a 

 vast boulder formation. 1 In ascending the valley, the 

 plain which at the mouth on the southern side is 355 

 feet high, is seen to trend towards the corresponding 

 plain on the northern side, so that their escarpments 

 appear like the shores of a former estuary, larger than 



1 I have described this formation in a paper in the ' Geological 

 Transactions,' vol. vi. p. 415. 



