Yol. 59-] GEOLOGY OF. PATAGONIA. 175 



the Fenix makes a sharp turn to the north-north-east. Still farther 

 east are more pampas, which ultimately ahut on an extension of the 

 basalt-plateau not far beyond Laguna Paje. 



The history of Lake Buenos Aires may be read from the evidence 

 afforded by the shores thus briefly described. First, at some date 

 subsequent to the basaltic eruptions and prior to the extensive 

 glaeiation, a huge transverse valley with an approximately east- 

 and-west trend was formed by a series of faults. It would have, 

 been safe to assume this, even if direct evidence had not been seen, 

 for no one would venture to attribute such a chasm to river-erosion. 

 But direct evidence is there, in the isolated mass of basalt, occupying 

 less than half a square mile of the delta of the Fenix ; a cliff 

 bordering the lake 50 feet high; a roche moutonnee slightly 

 lower ; and several glaciated pavements hidden among the bushes. 

 There is no other basalt in situ visible nearer than the great plateau 

 8 miles to the south, which is over 3000 feet higher, therefore it 

 cannot be looked on as an older and distinct flow ; indeed there 

 seems little reason to doubt that this is a remnant of a large faulted 

 area of the plateau. AVhat it was that caused this valley to be 

 filled with water cannot be definitely stated, seeing that no geologist 

 has yet visited the western end of the lake ; but, taking into considera- 

 tion the greater extent of glacial conditions in former times, and 

 the presence of large glaciers to-day on Colle San Valentin and 

 Colle Arenales, it is a reasonable conjecture that it was owing to 

 glacial ponding on a large scale. Dr. Moreno 1 speaks of the lake as 

 a fjord, an expression which must be taken to imply that he considers 

 it to be an arm of the Pacific raised above sea-level. Although the 

 resemblance of the narrower portion to a fjord is striking, no mention 

 has yet, we believe, been made of marine shells on the terraces or 

 on the shores. No doubt the general elevatory movement in Chile 

 and the Argentine affected the area in question, but it was probably 

 owing to the Cordillera being unable to bear the strain of that 

 movement at this particular point that the transverse, and also the 

 longitudinal, valley was formed, so that the depression may not have 

 acquired its greatest development until some period had elapsed 

 after the first upheaval, the entrance of the ocean being thereby 

 rendered unlikely. 



The terraces mentioned by the same author in the western, 

 and those seen by me in the eastern, portion of the lake, together 

 with four raised beaches identical with that now at the head of 

 Engineer's Inlet, show that the original level was higher than at 

 present, and has been gradually decreasing. Its greatest extent 

 has not yet been determined ; but since, as Dr. Moreno has shown, 

 the ancient outlet, in common with those of other Patagonian lakes, 

 was to the Atlantic, the height of the land on the east shows that it 

 may have been 700 feet higher than at present. The breaking-down 

 of the barrier at the western end of the lake would cause the waters 

 to fall to successive levels and prevent them at the same time from 

 flowing eastward, diverting them instead to the Pacific. 

 1 Geogr. Journ. vol. xiv (1899) p. 354. 



