Fenton — Physiography and Glacial Geology of S. Patagonia. 209 



least in the lower seventy miles of tlie river valley; on the other hand, the 

 lava lies on the shingle in more than one place, and I have reason to believe 

 that it does so in every instance. This would prove the lava to be the 

 younger rock of the two, so that, if tlie lava corresponds with a I'esting phase 

 of the river valley between two phases of ice and flood, the shingle must have 

 come at the end of the flood or before the resting phase. The history of a 

 river valley in this region may be divided into four periods as follows : — 

 first, an ice phase; second, a flood-cutting phase; third, a flood-depositing 

 phase : and fourth, a resting phase. If now each terrace marks the base of a 

 former river valley, we should expect this sequence of events to have been 

 repeated as often as there are river terraces. 



The further discussion of this very important and fascinating subject I 

 must leave until I am in a position to obtain additional evidence. It seems 

 clear that a great period of ice-advance was followed by a long period of 

 erosion by water ; this flood period was intimately connected with glacial 

 action, and the huge spring and summer floods were due to a much greater 

 winter snow-fall in former times. 



There is a terrace along the Gallegos river, which is covered for a 

 considerable distance (half a mile or more) with a mass of broken debris, 

 consisting chiefly of huge angular fragments of basalt up to six feet or more 

 in length. Packed between these basaltic fragments, will be found a certain 

 amount of pampa shingle, many of the stones of which are wedged in such 

 a way that it is clear that they came there at the same time and with the 

 basaltic fragments. This mass of debris lies on a lava-sheet, which shows a 

 diff'erent composition and structure from the great basaltic blocks above, and 

 the whole mass of this deposit was evidently carried from another locality, 

 and heaped up over the lava-sheet in question. A little further up the valley 

 the river makes a well-marked bend round to the north, and here, on the 

 terrace, is a peculiar knob standing by itself. This is the remnant of a 

 lava-sheet, which once covered this terrace up to the level of its top. It is 

 about thirty feet in height, and is removed upwards of half a mile from the 

 nearest edge of the sheet, of which it originally formed a part. Hence 

 extensive surface-erosion of lava occurred here at one time, and it is more 

 than probable that it was from this surface that the lava blocks were derived 

 which we saw heaped up on the lava-sheet below. We see from this that 

 during the cutting down of the river valleys there are two forms of erosion 

 of lava — namely, the ordinary erosion of the sides of the sheet during the 

 cutting down and deepening of the river valleys, and also a form of surface- 

 erosion, where great areas of the surface of a sheet were broken up and 

 carried away in huge fragments. A few miles down the river, at a place 



