Fenton — Physiography and Ghicial Geology of S. Patagonia. 217 



miles broad and thirty miles or more in length none the less presents striking 

 analogies with the miniature hollows eroded in the floors of ordinary 

 streams. It may be once more pointed out that the thickness of the mass of 

 stagnating ice resting on the pampa provided a very effective head for the water 

 pouring in Niagara-like cascades from its surface. I have also found in one 

 place a series of bajos extending in an irregular line from west to east for 

 thirty or more miles. In some instances the bajos are found opening one 

 into the other, while in others a narrow strip of intact pa-mpa may separate 

 them. Such series often occur, I am informed, in other parts of the country 

 also. 



IX. 



The Buitreixis Bed. 



As I have already said, the lava in this part of Patagonia is practically 

 always found lying superficial to the shingle, and consequently it is believed 

 to have been poured out after the pampa shingle was distributed. I was 

 consequently somewhat startled to find well-marked sedimentary rock 

 between two layers of lava running out from the cone shown in fig. 3, which 

 is situated at the side of the Gallegos river, about fifty-five miles from its 

 mouth, at a place called the Buitreras. On close examination, however, it was 

 found that this rock differs considerably from any of the tertiary deposits. 1 1 is 

 of a coarse sandy grain, generally of a yellowish colour; in some places it would 

 seem to consist of volcanic tuff, and in most places is stratified in thin layers 

 from one to a few inches in thickness ; but the most striking fact is that it 

 has not only an occasional angular fragment of lava embedded here and there 

 in its matrix, but also a considerable number of typical pampa pebbles. These 

 pebbles, in the exposure at the side of the Buitreras cone, are in all the 

 varieties and sizes generally seen on the pampa, and are embedded in the rock 

 in all the levels which here crop out. About a mile and a half up along the 

 same side the high river bank juts out at right angles to its main course for a 

 short distance. The top of the bank is here about 170 feet above the river, 

 and consists almost exclusively of material similar to that already described, 

 except that there are neither pebble nor lava blocks embedded in it. It also 

 lies above and below a sheet of lava which crops out more or less in its middle. 

 Along this same bank a few hundred yards further the ground rises for a 

 short distance to the level of the main pampa, and shows the usual pampa 

 shingle on the top ; in fact, it has all the characteristics of an isolated flat- 

 topped hill, and its face, as it borders the river from summit to base, consists 

 of the same class of rock found opposite Gallegos and along the Atlantic 

 coast. In other words, it is typical tertiary rock. At the same place where 



