388 



AMAZONIA AND LA PLATA. 



shoot tliese rapids at a velocity of 12 or 15 miles an hour. The Santa Cruz is 

 certainly the most copious of all the Patagonian rivers, and Moyano estimates its 

 discharge at no less than 30,000 cubic feet per second. It draws all its supplies 

 from the lakes, for the little rain that falls farther east is absorbed in the sur- 

 roundins: volcanic scoriae. Its channel and the lateral terraces are strewn 

 with erratic boulders, huge masses of 17,000 or 18,000 cubic feet. Assuming 

 that in the Upper Santa Cruz valley the average annual rainfall amounts to 

 about 30 inches, a figure which seems approximately correct, the superficial 

 area of a catchment basin necessarv for the development of such a stream 



Fig. 158. — Moirnis op the Rios Chico and Santa Cruz. 

 Scale 1 ; 1,100,000. 



r" 







50' 



V f 







West opGreenv'ich 69* 



Depths. 



too 

 Fathoms. 



5 to 10 

 Fathoms. 



10 to ys 

 FaihoiMS. 



2.5 Fathoms 

 and upwards. 



6 Miles. 



as the Santa Cruz should exceed 13,000 square miles. Such in fact must be 

 the extent of the region from which Lakes Viedina and Argentiuo draw their 

 supjDlies. 



At its eastern extremity the fluvial valley, bordered by cliffs from 100 to 

 400 feet high, has quite the aspect of an ancient marine strait, and Darwin sug- 

 gested the idea that it might have at one time formed a passage between the two 

 oceans, like another Magellan Strait. This hypothesis, however, is not supported 

 by the aspect of the mountains rising to the west of Lake Argentino. 



In the estuary converges ariOther river, the Rio Chico, which is often re- 



