EOTLicH.] VOLCANICS. 199 



only a few places on the river. The general clip of the flows to which 

 the blnft's owe their existence is a little south of east, amounting from 

 2 to 5 degrees, except at points where later local disturbances have in- 

 creased it. 



At Wagonwheel Gap a local phenomenon of some interest occurs. 

 The flow of trachytic material became somewhat columnar upon cooling, 

 and afterward a small hill composed of it was torn apart, leaving 

 nearly vertical clilfs ou either side. Through this narrow passage the 

 river has found its way, leaving on either side of its banks suflicient 

 space for wagon-roads. Indians have taken advantage of the com- 

 manding view obtained from the highest portions of the hill, and numer- 

 ous abandoned "lookouts" and low walls along the edges testify to 

 their presence in former days. At that locality, as well as several 

 others, the trachyte No. 2 contains numerous fragments of jasper, 

 chalcedony, and flint, sometimes in the form of geodes. Above the gap 

 the same formations continue without any material change in either 

 stratigraphical or orographical features. The valley of the Eio Grande 

 all along there is of considerable width, and the bottom composed of 

 trachytic drift. To the south and southeast the La Plata Mountains 

 rise to considerable elevations, consisting probably of trachytic material 

 also. 



Antelope Park, at an elevation of 9,000 feet, -presents some interest- 

 ing ieatures. To the northeast of the park is Bristol Head (station 54), 

 the termination of one of the long, high plateaus running southward 

 from station 2. At its southwest termination this plateau presents a 

 vertical wall over 2,500 feet in height; then proceeding farther in that 

 direction, the Santa Maria lake is reached, while on the other side of it 

 there is an analogous vertical wall, about 1,200 feet in height. The lake 

 is contained in a long narrow valley, partly timbered, partly grasvsy. 

 At the east end of this valley are the Antelope Springs, so called. Upon 

 examination it will be found that the upper strata of trachytic rocks ou 

 the bluff edge upon which the station was located, correspond to those 

 on the other side, although they are more than 1,000 feet lower down. 

 It is a case of sudden subsidence, whereby the portion on the south- 

 western side changed both its horizontal and vertical position, while 

 Bristol Head remained stationary. To the northeast of the station the 

 strata dip slightly in that direction, while those having fallen, on the 

 other side, dip at an angle of 6° to 10° to the southwest. This dip 

 shows that the subsidence was by no means a i3erpendicular one, but 

 that the southern side fell more than the opposite. Thanks to the ex- 

 cellent view obtained Irom Bristol Head, there could not very long re- 

 main any doubt as to the action that produced this result. Eeachiug 

 Antelope Park from the west side, an exposure of the upper members of 

 2^0. 1 and the lower ones of the succeeding numbers will be observed. 

 Physically they are soft, and rapidly yielding to atmospheric and 

 erosive influences. Opposite that exposure, which is soon hidden from 

 sight by the secondarily acquired position of the "dropped'' superincum- 

 bent beds, the Rio Gtande emerges from a narrow canon, and closely 

 hugging the rocky batiks on the south side of the^jark, follows them for 

 soQie distance below San Juan City. The valley comprising the park 

 is slightly rising toward Bristol Head, and in it the old course of the 

 river can easily be traced. It will be found that instead of following 

 closely 'along the south banks, which are determined by the trachytic 

 bluffs of No. 2, the river from its ])oint of egress formerly flowed acrosa 

 the entire valley, and, winding along in curving lines, kept a course 

 much nearer to Bristol Head than the one it has to-day. It seems 



