176 MB. J. B. SCEIVENOB ON THE [May I903, 



During the latter part, at least, of the history of Lake Buenos 

 Aires, the lake and the slopes on either side were covered by ice 

 coming from the Cordilleras. The form of the roche moutonnee 

 and the scratches ou the glaciated pavements at the mouth of the 

 Fenix show the direction of its advance to have been due eastward. 

 This ice-sheet it was that deposited the vast accumulations of 

 morainie detritus when it melted ; moreover, it is due to the irre- 

 gular surface of the debris that numerous muddy pools are scattered 

 over the lower slopes. 



The two lakes, Musters and Colhuape, are situated in the 

 midst of the more northerly basaltic area, the former under the hills 

 believed to represent an old Patagonian mountain-chain, the latter 

 more to the east. A companion of mine saw Lake Musters and 

 gathered valuable information as to its topography. I myself, during 

 the short halt at the settlement of Colhuape, had perforce to con- 

 fine my attention to the eastern lake. Great doubt appears to 

 exist about the topography of these two lakes and their rivers, even 

 to the extent of the names of the lakes being interchanged. What- 

 ever may have been the original design of the discoverers, there is 

 no doubt that now the inhabitants of the Welsh settlement there 

 and in Chubut call the eastern lake Colhuape, the western Musters. 

 So far as could be gathered from personal observations and local in- 

 formation, the following are the leading features: a ridge, extending 

 from below the southern shore of Colhuape northward, cuts off the 

 one lake from the other ; there is no communication between them 

 visible. Musters is fed by a river entering it at the north, the origin 

 of which is not clear. Lago Colhuape is supplied by the Eio Senguerr, 

 which, flowing from the Cordilleras at first with a south-westerly 

 course, makes a bold sweep round the hills to the west of Lake 

 Musters, and then, leaving that lake on the left, flows on into 

 Colhuape. No outlet for Lake Musters is known, but Colhuape is 

 drained by the Eio Chico de Chubut, on the east. 



Lago Colhuape is fast being silted up. Unlike that of Musters, 

 its water is muddy and shallow, and already one- third of the basin 

 in which it lies has been converted into a rich alluvial flat, on which 

 the Welsh and other settlers have established themselves. The 

 form of the lake does not suggest an old river-valley. It lies in a 

 vast amphitheatre, overlooked on the east by the broken edge of a 

 basalt-plateau, and on the west by the ridge that separates it from 

 Lake Musters and the ancient mountain-chain beyond. If ever the 

 physical geology of these lakes is worked out in detail, it will not 

 be surprising to me to find that they also owe their origin to tectonic 

 disturbances of the same date as those which formed Lake Buenos 

 Aires. 



VIII. The Rivee- Valleys. 



The valleys of the rivers which flow across the pampas are 

 all clean-cut troughs, the walls of which rise to the flat land above 

 at an angle varying from 30° to 40°. Terraces, well marked on 



