192 REPORT UNITED STATES GEOLOGICAL SURVEY. 



summits. The trachyte of which they are formed seems to have reached 

 its present place through a number of channels, although probably 

 from the same nucleus. The masses now exposed were doubtless forced 

 up through narrow crevices or apertures until the yielding formations 

 of the Middle Cretaceous were reached, where the melted material 

 spread out to the right and left in great masses and sheets. The shales 

 are still found in all parts of the group, caught up in a manner identical 

 withthatobservedintheLateandOarriso Mountains, and the low saddles 

 between the various groups of summits are invariably of these shales. 



All along the east and south bases of the mountains the sandstones 

 of the Dakota group are turned up at high angles. Near camp, at the 

 base of the primary triangulatiou station, a double row of hogbacks 

 composed of these sandstones skirt the flanks of the hills. They are 

 somewhat metamorphosed and have a dip of 45°. The slopes above these 

 outcrops are for the most part covered with grass and bushes or with 

 loose slides of finely broken trachyte. In a number of places, however, 

 there are outcrops of variegated shales and slates, and toward the sum- 

 mits considerable masses of trachyte protrude. 



The tendency in all parts of the mountains is to weather into smooth, 

 rounded forms, owing, perhaps, to the homogeneous and rather slightly 

 compacted character of the trachyte. Eemarkable uniformity is notice- 

 able in the height of the various summits, and some are nearly flat- 

 topped. As their general height above the surrounding floor of the Da- 

 kota sandstones represents so nearly the thickness of the Cretaceous 

 shales, the idea is suggested that possibly the great mass of trachyte 

 did not penetrate the very massive sandstones of the Upper Cretaceous, 

 which at that time must have covered the whole region. 



There are four principal groups of mountains which, together, cover 

 an area of some 150 square miles. Their arrangement is such that the 

 total extent is a little greater northwest and southeast, but the trend is 

 not sufficiently marked to make a well-defined range. The southeastern 

 section is the only one that comes within the limits of the Colorado sur- 

 vey. This is the group usually seen from the east, and its character will 

 be understood by reference to Plate XIII. Our station was made on the 

 central summit. At the south end is a prominent point almost as high 

 as our station, which presents a sharper summit and more abrupt faces 

 than any other of the entire Sierra. 



The drainage of the north and east faces of this group belongs to 

 West Montezuma Creek, while that of the south and west is appar- 

 ently tributary to western branches of the same stream. The extreme 

 head of the Montezuma seems to be northwest of our station, between the 

 second and third groups of mountains. From this point it circles around 

 to the northeast and south, passing near the north edge of the Sage 

 Plain opposite the head of Cold Spring Canon. The canon begins at a 

 point near the east base of the mountains and rapidly deepens toward 

 the San Juan. In the middle part it is upward of 1,000 feet in depth, 

 and the red rocks of the Jura-Trias are exposed. Between the base 

 of the mountains aad the Montezuma CaQon there is a considerable 

 area occupied by the Cretaceous shales. East of the canon the shales 

 only occur on the higher and more protected spots. 



Although the country which surrounds this group of mountains is 

 desert-like in the extreme, there is a narrow belt about the flanks and 

 lower slopes that abounds in vegetation. Considerable forests of piiion 

 pines occur about the canoned region to the east and south. Large 

 groups of these trees are also scattered at intervals over the greater 

 parts of the Sage Plain. Groves of large yellow pines skirt the imme- 



