216 Scie)itific Proceedings, Royal t)uhlin Society. 



The great difficulty in the way of accepting the foregoing theory in its 

 entirety is the presence of the shingle ; and it seems unthinkable to me that 

 wind, even assisted by water, could produce this great erosion of a surface 

 protected by a deposit of fifteen feet of pebbles, many of them up to eight or 

 nine inches in length. Then it may also be asked why should this erosive 

 action only affect isolated patches when the great portion of the area is 

 untouched ? The tertiary rock below is of a clayey nature, and quite 

 insoluble ; it would consequently not suffer any erosion under fifteen feet of 

 shingle and sand. If by any chance certain patches of the pampa remained 

 uncovered when the shingle was washed down from the first great ice-sheet, 

 these patches would then be exposed to the action of wind and water, and' 

 bajos would soon result. I was talking this matter over with a friend one 

 evening, and he suggested that perhaps there might have been a few small 

 hills remaining in the first instance which had resisted the great planing 

 influence which was operating in pre-shingle times, and that these hills, 

 being higher than the surrounding plains, were left uncovered when the 

 shingle was washed over the country generally. This explanation would 

 account for all the facts observed, and I now beg to put it forward, with all 

 due reserve, as a working hypothesis. There is, however, one serious objec- 

 tion to this idea : there is a hill, or rather a small tableland, situated on the 

 main pampa to the north of the Gallegos river ; it falls all round, particularly 

 towards the west, to the extent of over forty feet, and yet it is covered by 

 the same shingle, and to the same extent, as is found on the rest of the 

 plains. Jt may consequently be asked why should slight elevation above 

 the surface render the one set of hills immune from shingle deposit, when 

 this plateau, which is fully forty feet above the main plain, is covered com- 

 pletely ? 



There seems little doubt that some of the bajos must have had their 

 origin well back in inter-glacial times, as can be seen from their huge extent, 

 the fact that there is more than one horizon in them, and that, as climatic 

 conditions prevail at present, they do not seem to be suffering any 

 marked erosion. Since they are found on all the river-valley horizons, 

 except the lowest, factors would seem to have been present for their produc- 

 tion in all the inter-glacial periods ; these factors, moreover, were evidently 

 absent since the last retreat of the ice. 



There is yet another solution of the problem that I regard as the most 

 satisfactory of all. The stagnating ice-mass postulated in Section VII would 

 give rise to localized and often broad-fronted waterfalls during its epoch of 

 rapid melting. The bajo may be regarded as a large pot-hole, or, rather, a 

 vast representative of the pools in a stream with occasional waterfalls. Such 

 pools are often deep at the head and shallow at the foot, and a bajo fifteen 



