Fenton — Physiography and Glacial Geology of S. Patagonia. 219 



river was dry. We find that the Buitreras heds have in places been cut down 

 a certain distance, when there was a volcanic outbreak and a lava sheet was 

 poured over them ; a terrace which had been formed at the mouth of Gallegos 

 Chico was in its turn cut further down by a subsequent action. This extensive 

 terrace abuts, not only on the Gallegos Chico, but on the Gallegos river also, 

 and indicates that there were at least two periods of cutting and probably 

 one of glaciation since the Buitreras beds were formed. Further study of 

 this interesting deposit shows that it has suffered extensive erosion since it 

 was laid down ; in fact, only a few isolated patches remain here and there at 

 the sides of the huge valley that was once filled with it. It must have been 

 upwards of three hundred feet thick, and have extended for at least fifteen 

 miles. At the side of the Gallegos Chico (a small river which joins the 

 Gallegos proper at a place called Bella Vista) the Buitreras beds were cut 

 down some 150 feet, and a sheet of lava was poured over them. This 

 latter was farther cut through by the Gallegos Chico river another eighty feet, 

 until the tertiary rock was reached and the floor lowered to its present level ; 

 and then, as I will show later, a glacier descended into this last river valley, 

 and filled its mouth completely with a terminal moraine. Finally, this 

 moraine has now in turn been cut through by the Gallegos Chico as it runs 

 to-day. The past history of this valley was, then, something as follows : — 

 First, the great shingle period, then a great period of erosion by floods, which 

 may or may not have been intermittent; this lasted until the river valley 

 was cut down to almost its present level. Then occurred a long period of 

 deposition, which was probably intermittent, and during which there"was at 

 least one considerable outburst of volcanic activity. When this deposition 

 had been completed, and between two and three hundred or more feet of rock 

 had been formed, there occurred once more a long period of erosion by water, 

 which probably had two or three breaks, as, although there is only one 

 terrace to be seen in the valley of the Gallegos Chico, yet a little lower down 

 there are two more or less distinct terraces. Now, if each of these inter- 

 missions of erosion occurred in the manner already described, it appears that 

 there were a considerable number of advanced recessions of the ice in this 

 valley. The cause of the deposition producing the Buitreras heds is obscure. 

 There is no trace of a dam across the river valley lower down which might 

 have caused a great lagoon behind it. If during a subsidence the sea came 

 further up the river valley, the current of the flood-water would have been 

 considerably slackened, and sedimentation would have taken place; a bar 

 might have also been formed and have helped the sedimentation. I did not, 

 however, find any trace of such bar or any evidence of marine formation, so 

 I have been consequently compelled to defer the further consideration of the 

 subject until I am in a position to obtain more evidence. 



