214 Scientific Proceedings, Royal Dublin Society. 



half a mile to a mile in length, can be seen. Some of these caiiadones are 

 more like arms of the bajo, as the main floor-level of the bajo extends into 

 them for almost their whole extent, or at least the floor of these arms is 

 often not much higher than the main floor. This bajo was formed by 

 erosion, and not by subsidence, since the strata of tertiary rock crop out on 

 the sides, and are clean cut off at their edges, as they are on the cliffs along 

 the Atlantic coast. As a rule, between the bights already mentioned ridges 

 extend downwards and outwards into the bajo from the top. In some 

 places these ridges form three irregular steps ; these latter, however, 

 although a rough attempt at terracing, are not anything like so well marked 

 as the terraces along the river- valleys. In some places the ridges are fairly 

 broad, whereas in others the sides of the bajo are cut every hundred metres 

 or so by in-running cariadones. The lower terrace is in one place so cut that 

 it shows two or three little rounded hills, with flat tops, these being so equal 

 and even in height that they give the idea of their marl<ing the level of the 

 floor as it existed at a former time. The pampa on all sides of the top of 

 the bajo is equally level, and on the east side there is no more sign of a hill 

 than there is on the west, so that of the great mass of material which once 

 filled up this hollow not a trace remains. The three salt lagoons, from 

 which the bajo takes its name, only occupy a small portion of the floor, the 

 remainder being covered with bush, grass, and other vegetation. The caKa- 

 dones and bights, already described, leading into this bajo are very similar 

 to the caiiadones found elsewhere; they are clothed in vegetation, and show, 

 like the others, practically no signs of present-day erosion. Hence it is 

 probable that the Bajo de las Tres Lagunas has not altered much during 

 recent centuries. 



Now, all the characteristics of this bajo are to be found also, to a certain 

 extent, in all the others. Half a mile to the eastern end of this great 

 valley there is another one, which is very much smaller in size and depth ; 

 this latter is about three-quarters of a mile long, and about three hundred 

 yards broad, and about eighty feet deep ; at its western end a bare, gravelly 

 patch of a few acres in extent marks the situation of a small lagoon, which 

 exists, as a rule, only in spring and early summer. The centre of this patch 

 is covered witli dry mud, and there is no sign of vegetation on any portion 

 of it. There is no attempt at terracing in this bajo, and it would seem to 

 consist of one horizon only, although, like the other, its lower portion is the 

 western end, and it slopes sligjjtly upwards towards the east. All the floor 

 not occupied by the bare patch already mentioned is covered by the same 

 vegetation as is found elsewhere, and shows no sign of recent erosion. 

 Everywhere over the pampas semi-bare patches occur, which are often only 



