Fenton — Physiographij and Glacial Geology of S. Patagonia. 201 



erosive effort of the last phase of the Ice Age. If this is the case, it will be 

 evident that the lava ontpoiuing of the Cerro Diablo is at least post-Glacial. 

 The other lava-sheets found on the pampas differ considerably from the 

 above. There are no gaping fissures and no open holes to be seen anywliere 

 in the rock, all such having been long ago filled up and obliterated; and 

 there is no intact cuticle, nothing being found but fractured surfaces. Also 

 where they reach the edge of a river valley or canadon the sheets break off 

 abruptly, and fall in the form of cliffs. All over such lava-sheets a fair 

 amount of recent soil occurs, with abundance of good grass and vegetation ; 

 in fact, the hommocky masses already referred to are in many places only 

 seen here and there jutting through the grass. 



Except in one or two trifling cases, which I will return to later, no shingle 

 is found on any of the lava-sheets ; the soil in which the grass and bushes grow 

 is a rich, black loam, and does not contain any pebbles in its matrix. In the 

 areas between the lava-sheets, where large erratics are lying about here and 

 there all over the surface, none of these erratics consists of lava. Conse- 

 quently the period of volcanic activity must have had its origin subsequent 

 to the termination of the first great Ice Period. In the summer of 1914 

 I was fortunate enough to find two places where the typical pampa shingle 

 could be seen under the lava, and, more strange still, in one of these places 

 I found that between the shingle and the lava there was about one hundred 

 and fifty feet or more of a well-stratified rock. This latter, which is of the 

 greatest interest, I hope to describe in a subsequent section of this paper. 

 It was formed long after the shingle was deposited on the pampas. Con- 

 siderable erosion has occurred in many of the early lava-sheets since their 

 formation. In one place a canadon, some hundreds of yards wide, and eighty 

 to a hundred feet deep, has been cut through them. 



A feature of importance in the present study is that the lava- blocks are 

 often to be found arranged in a peculiar sloping manner, rising towards the 

 east; and on the slope facing west there are usunlly to be found a number 

 of grooves running up along the stones in an oblique manner. The space 

 between the grooves has always a smooth, ground-down appearance, and the 

 surface is often concave. This grooving and the peculiar lie of the stones are 

 well marked in PI. VI, fig. 2. This peculiarity of the lava-blocks is found in 

 widely different localities ; the grooves always run in the same oblique manner 

 from east to west, and always on the sides of the slopes facing west. The 

 grooves may be seen on the north or south sides of the stones, but are never 

 found on the eastern ends. The direction varies from due east to east-north- 

 east. During several years of residence, I have visited every possible lava- 

 sheet that I could approach in the Gallegos district within eighty miles of the 



