FENTON—Phi/swffraph?/ and Glacial Geology of S. Patagonia. 211 



shaped flat lava-topped piece of ground, situated on the south bank of the 

 (rallegos river, opposite the Buitreras cone. It is about two miles long by, 

 perhaps, half a mile broad. It is bounded on the north by the Gall egos 

 river, and on the south by a large canadon over two hundred yards in 

 breadth, which separates it from another extensive lava-sheet to the south. 

 It falls abruptly to the west, but gradually to the east. The lava ridge is 

 here about ninety feet high above the river valley, and the lava-sheet, which 

 is still practically intact on the western end, is completely broken up on the 

 eastern portion. On carefully examining it, we will find that a peculiar ridge 

 of more or less intact lava extends down as a fringe along the northern edge 

 of the table for a little distance. One of the above-mentioned blind 

 canadones extends up behind this ridge to the south. The Buitreras table 

 has thus suffered a considerable amount of surface-erosion during late glacial 

 times ; and, although the erosion is almost altogether confined to the eastern 

 two-thirds of the table, the eroded area encroaches on the intact western end 

 in the form of a series of bights into the lava-sheet. There are also some 

 hollows eaten out of the latter, which end blindly in all directions. What 

 then was the agent which produced this condition of surface-erosion, which 

 is by no means limited to this particular place ? There are no signs of sea- 

 shells, or any form of beach ; besides, all the great basaltic fragments are, as 

 a rule, angular, and not rounded as they would have been if the sea had been 

 playing on them for any length of time. A great flood might have been the 

 cause ; but such is difficult to imagine. What then was the state of affairs in 

 the Buitreras area in the particular time that we are considering ? 



I have imagined to myself a great mass of snow gradually settling by 

 pressure into ice, accumulating for centuries generally over the whole country, 

 particularly on the high pampas. This went on to such an extent that the 

 river valleys became obliterated by ice. The country was so flat that this 

 ice-sheet had no tendency to move, but simply lay there as a stagnant mass 

 in the area in which it was formed. At least its movement was so slight 

 that it caused no moraine formation, and no scratching of the stones. As a 

 consequence of the same stationary condition, the ice-mass was clean, and 

 held practically no picked-up material, except perhaps a little in its lower 

 stratum. Then occurred a change of climate, which set in rather rapidly, 

 with the result that this great ice-sheet began to melt away. Hot summers 

 set in, during which huge floods occurred, masses of water poured down 

 through large crevasses, producing extensive sub-glacial erosion, and from the 

 eastern fringe of the ice-mass immense blocks of ice became broken off, and 

 floated away to sea on the waters. As they broke away, they carried with 

 them large pieces of the lava, which separated along with them. When the 



