SIERRA VENTANA. II? 



seem quite as well adapted to their needs as those to the north, but the 

 entire southern half of the valley of that river is especially well suited 

 to them. Though common in the valley on the north side of the river, 

 no example has ever been taken to my knowledge in the valley on the 

 south side. The temperature of the water in this stream, its great size, 

 and the absolutely treeless nature of the entire region through which it 

 flows render it particularly capable of presenting an effective barrier to 

 the free migration of certain mammals, and more especially those like 

 Tatusia, which are probably not capable of swimming and are known to 

 hibernate in winter, at which period alone they would be able to cross 

 such a stream on the ice. Their flesh is of an excellent flavor and highly 

 prized by the natives as food. 



On the evening of the twenty-first of January we camped at a small 

 spring on the western border of a bad-land area of considerable extent. 

 An hour or two spent in examining the various exposures in the neigh- 

 borhood of our camp resulted in the discovery of sufficient material to fix 

 the deposits as belonging to the Santa Cruzian formation. 



On the twenty-second we crossed through these bad-lands, stopping 

 at various places to examine such localities as appeared especially promis- 

 ing for fossils. In the evening we camped at a nice spring on the south 

 side of the River Chico and some three miles above Sierra Ventana, a 

 photograph of which is shown in Fig. n. This mountain, which is in 

 fact the core of an ancient volcano, stands directly on the south bank 

 of the river, above the valley of which it rises to a height of some twelve 

 hundred feet During the afternoon we ascended this gigantic mass of 

 basalt and cinders and stood on what still remained of the rim of the 

 crater which crowns its summit From the bed of the river the first five 

 hundred feet or more of the slope consists of sedimentary materials be- 

 longing for the most part to the Santa Cruzian formation. The surface 

 of this lower part of the slope is almost entirely covered over with great 

 blocks of columnar basalt, and quantities of fine cellular pumice that have 

 fallen down from above. At an altitude of some six hundred feet from 

 the base the giant columns of basalt, which now fill the former vent of the 

 volcano, rise out of the surrounding sedimentary rocks to an additional 

 height of some four hundred feet These columns gradually converge 

 toward their summits, as shown in the photograph reproduced in Fig. 1 1, 

 as though there had been a constriction in the neck of the volcano 



