Vol. 59.] THE GEOLOGY OF PATAGONIA. 173 



Tehuelche Pebble-Bed than is the case, and taken a large series of 

 observations to that end. But it may be said that the general 

 impression left on my mind is that there is a great irregularity in 

 the size of the pebbles ; that there is no definite increase or decrease 

 in any direction ; and that, if these pebbles were called to witness 

 to the course of an ancient stream, they would only mislead. 



Becent marine shells were seen on the pampas and elsewhere at 

 various points : a Patella at Dos Pozos, between Chubut and Came- 

 rones, with the colour preserved ; two more at Monte Leon, at an 

 altitude of 325 feet ; and two big specimens of Voluta on the banks of 

 the Rio Santa Cruz, 18 miles from its mouth. Yet I saw no occurrences 

 which entirely precluded the possibility of human agency, although 

 such an explanation is improbable. However, anyone who has 

 read Darwin's ' Geological Observations ' cannot doubt that there is 

 sufficient evidence in South America for the elevatory movement 

 necessary for the formation of the Tehuelche Pebble-Bed by marine 

 agency. 1 



VII. The Drainage-System. 



The drainage-system of Patagonia, comprising the rivers that flow 

 eastward over the pampas and innumerable lakes, vast basins like 

 Lago Buenos Aires not only penetrating into but even piercing the 

 Cordilleras, moraine-ponded lagunas, pampa-pools (fresh and salt), 

 and the two isolated lakes Colhuape and Musters, affords a study 

 as varied as it is interesting. For economic purposes the supply 

 under the Cordilleras is abundant, but north of the Rio Santa Cruz 

 and east of the foothills it is very poor. Springs are so scarce on the 

 pampas inland that travelling across them is very difficult, and even 

 impossible in places without guides, when once the snow-pools have 

 dried up. Near to and on the eastern coast, however, there is an 

 abundance of small streams welling from under the steep banks 

 which lead from one pampa to another, a fact which makes sheep- 

 farming possible along a narrow belt bordering the Atlantic. 



Of the lakes, Lago Buenos Aires calls for description, not only 

 because it is the largest in Patagonia — being 75 miles long and, in 

 the eastern portion, 12 to 15 miles broad — but also because it lies 

 in an enormous transverse valley cutting through the Cordilleras, and 

 drains into the Pacific by a stream connecting with Lake Soler, 

 which is in turn drained by the Bio de las Heras. This drainage to 

 the west is all the more remarkable, since by far the greater part 

 of the water is supplied by rivers rising on the eastern side of the 

 Cordilleras. The height of the lake, as ascertained by Seilor Waag, 

 of the Argentine Boundary Commission, is only 761 feet above sea- 

 level. Seeing that on either side there are peaks exceeding 12,000 



1 Mr. Hatcher has noted a large erratic block in the valley of the Cbico de 

 Santa Cruz, close to the junction with the Chalia or Sheuhuen; the nature 

 of the block is not mentioned. It may be suggested that the granitic and 

 gneissose pebbles have been brought to the eastern coast by floating ice ; but the 

 number of these pebbles, their regular distribution, and, in my experience, the 

 total absence in the Santa Cruz district of exceptionally large pebbles of any 

 sort in the Pebble-Bed strongly militate against this theory. 



