VALLEY OF THE RIO CHAI l\ 115 



place where the stream, with low banks on either side, flowed over a bed 

 of coarse shingle in such manner as to offer an excellent natural ford, we 

 crossed safely to the other side, where, with an abundance of grass and 

 pure water from the stream, we camped for the remainder of the day and 

 night, in order to grain for ourselves and horses that sleep and rest we so 

 much needed after the experiences of the preceding night During the 

 afternoon Mr. Peterson shot and skinned a few birds, while I, with my 

 plant press, proceeded to a rocky ledge that projected out into the valley 

 from the east at a distance of a few miles below. I found this to consist 

 of a dike of hard basaltic rock protruding from the surface at right angles 

 to the nearly horizontal strata of the surrounding sedimentary deposits. 

 Just above where we were encamped for the night the valley of the 

 river curved sharply to the westward and opened out into a broad basin. 

 On the western and northwestern border of this we could see extensive 

 exposures which, from their colors, evidently belonged to formations other 

 than any of those we had previously examined. On the following morn- 

 ing, January the nineteenth, we moved some fifteen miles farther up the 

 river, and spent the afternoon in an examination of the exposures men- 

 tioned above. We found these to consist of several hundred feet of 

 rather coarse, brown and variegated sandstones barren of fossils except- 

 ing a few uncharacteristic plant impressions. The following morning, 

 January the twentieth, was passed in studying some lower hills, a few 

 miles to the east. These proved to be made up, for the most part, at 

 least, of marine deposits belonging to the great Patagonian formation. 

 Returning to camp, we prepared an early dinner and left the Rio Chalia 

 that same day, travelling almost due north across a series of low pampas 

 and over the beds of several old dried-up lakes, that occupy depressions 

 in the surface of the former. Our course lay along the foot of a high 

 escarpment capped by a basaltic platform that rose to a height of from 

 six to eight hundred feet above the plain to the east, upon which we 

 were traveling. At night we camped at a small spring near the mouth 

 of a small caflon that emerged from the high, basalt-capped bluff on our 

 left and entered a deep dried-up lake basin that lay on our right The 

 spring afforded water just sufficient to supply us and our horses. Having 

 noticed some promising exposures about the shores of the desiccated lake, I 

 was up early the next morning, and, on examining them, found a number 

 of marine invertebrates that I had not previously noted elsewhere. After 



