212 Scientific Proceedings, Royal Dublin Society. 



floods were at their height, these lava-laden icebergs floated away to sea, as 

 I have said. However, when the floods diminished, most of the icebergs 

 grounded, and melted away in situ, shedding their lava debris. This 

 explanation seems the only one that accounts for the condition of affairs 

 found in this locality. 



If we now pass downwards towards the east, a few miles from the 

 Euitreras, the high pampa level to the south is seen to be capped to a con- 

 siderable extent with sheets of lava, which have been poured out from small 

 cones which surmount it close by. After we pass along the highest cliff, or 

 barranca, as it is called, for about two and a half miles below the Buitreras, 

 this lava-topped cliff takes a sudden bend to the south, at right angles to the 

 line of the river-valley. It runs in for about half a mile, and again runs 

 towards the east. During this half-mile course, the lava-cliff faces east, and 

 consequently has its back up stream. For several years I could not under- 

 stand how even a huge river filling the whole valley, here about five miles 

 wide, could turn this corner and undermine the lava-sheet to such an extent 

 as to cause this cliff. There is no sign whatever of a shingle beach at the 

 base, and all the fragments are angular, and show no signs of rounding, as 

 they would if the sea had been acting on them for a long period. On the 

 contrary, the whole place gives the impression of the cliff having been cut 

 down by a large mass of water pouring over it from behind. Now, this 

 particular cliff exists on the northern edge of the narrow tongue of pampa 

 which I have already mentioned as extending down between the Gallegos 

 river and the Eio Chieo. This tongue is extensively cut into, and forms 

 altogether a very small collecting area. The greatest rainfall which could 

 be imagined would not produce sufficient running water to cut down these 

 lava-sheets in the manner which is found to have taken place. A huge 

 accumulation of snow, piled hundreds of feet high on top of this table, and 

 then rapidly melting, might, however, produce sufficient running water to 

 bring about these changes. 



Now, if the process outlined above was the causative fact of the surface 

 conditions, the result would be that during a hot summer large volumes of 

 water would be formed by melting all over the ice-sheet. This water would 

 flow towards the east, and would fall over its limiting edge in an extensive 

 series of cascades. Now, at first, while the ice-sheet was still very thick, 

 these cascades, falling as they would from a considerable height, would 

 resemble a number of miniature Niagaras, and would produce much erosion 

 of the surface of the land on which they fall. They would also tend to 

 hasten the backward recession of the ice-sheet by breaking away its edge. 

 Huge fissures extending downwards through the ice as large blocks of the 



