Fenton — Physiography and Glacial Geology of S. Palagonia. 213 



latter became broken away and detached would also lead to local attack on 

 the underlying lava, which might in that way become very much broken. 

 A subsequent severe winter might now freeze lava-blocks and ice into one 

 compact mass to such an extent that the ensuing summer floods might pick 

 up large pieces of it and float them away. As, however, the ice-sheet as it 

 recedes is also all the time diminishing in thickness, a point will sooner or 

 later be reached when the ice-sheet will become so attenuated that its floods 

 are negligible, and have no further eroding action. Such a condition of affairs 

 was probably reached over the Buitreras table when the edge of the ice-sheet 

 reached the point already indicated near its western end. 



It must not be supposed, however, that the series of events which 

 ultimately took place was as simple as I have outlined above. It is much 

 more probable that during the whole action there were extensive oscillations 

 of temperature, not only from summer to winter, but also greater variations 

 when several seasons of comparatively hot weather alternated with other 

 long periods of cold. The idea which I have outlined seems to me the most 

 likely explanation of the phenomena which I have observed. 



VIII. 



The Prohlem of the Bajos. 



As I said in the first section, if we travel any considerable distance 

 over the pampas we ai'e sure to come across an occasional great hollow 

 which is not a river-valley. A typical example is the Bajo de las Tres 

 Lagunas, on the second pampas to the north of the Coyle river. The 

 pampa here is upwards of a thousand feet above the sea, and has all the 

 characteristic flatness already noted, so much so that we are almost on the 

 edge of the bajo before we are aware of its existence. This bajo is elliptical 

 in shape, with the long axis running east and west; it is about five miles 

 long, two broad, and the base of it is four hundred feet below the general 

 level of the plain. The western extremity is deeper than the eastern 

 end, and the three salt lagoons are situated in the western half. At the 

 sides of these lagoons, which in the summer and autumn are practically solid 

 salt, there are to be seen a number of springs ; some of these ooze up through 

 the mud, and are covered with a dry crust, which renders them very 

 dangerous, as animals often sink in, and are unable to extricate themselves. 

 The eastern end of the bajo is found to slope more gradually than the rest 

 of the circumference, which falls rather abruptly. As a rule there are no 

 very great canadones running into it; but a number of semi-cup-shaped 

 bights cut into the rim all along. However, in one or two places a canadon, 



