holmes! geology OF SEPULCHKE MOUNTAIN. 17 



bits of limestones and other sedimentary rocks. We have here the hig'h- 

 est outcrop of the pre-Tertiary strata that occur in this mountain. The 

 cliffs are capped by about 200 feet of gray and reddish-gray rhyolite. The 

 base of these rhyolites is at an elevation of 8,000 feet. This elevation 

 corresponds to the base of the great sheet of rhyolite that occurs on 

 Mount Evarts, and seems to indicate a former connection across the 

 valley of Gardiner River. Mineralogically the rocks are identical. 



The long flat ridge that lies between the wagon-road pass and" the 

 canon of the West Gardiner is capped by a sheet of hot-spring lime- 

 stone. This, however, is underlaid by rhyolite. In the pass we have 

 exposures of the Jurassic rocks, but. just beyond the rhyolites descend 

 to the bottom of the little valley that drains the western side. They 

 may be followed for a short distance along the base of the slopes to the 

 right and left of the road. A low ridge of dark rock reaches down to 

 the road from the left, and one at first sight is led to believe that here is a 

 mass of basalt, but on closer examination it is found to consist of a dark 

 pitchstone with large crystals of sanidin ; a rock similar to that found 

 forming the under surface of most of the rhyolite flows in this region. 



The peculiar topography of the little gorge or pass through which the 

 road is built leads us to inquire of its origin. It is not at all improba- 

 ble that at one time it has been occupied by a stream of water, which, 

 by some local change, has been turned into another channel. The high- 

 est part of the pass is composed of a loose mass of fallen rock and earth. 

 The bed of the channel of the supposed old stream may have been 

 choked up by such falling masses, or even hot-spring deposits, which are 

 so plentiful in the vicinity, may have assisted inthechange. There are 

 also some fragments of basalt and of drift material, which indicate that 

 this was a passage for water some time in the past. The courses of the 

 little streams that head about the west slopes of Sepulchre Mountain 

 are rather eccentric, and suggest the probability of their change by 

 some local cause from their original courses. The valleys that lie about 

 the west slopes of Sepulchre Mountain are covered with drift materials 

 and soil, and contain large areas of fine pasture lands. About the im- 

 mediate bases of the mountain there is a good deal of pine timber. 



From the pass I turned to the right and ascended the sharp spur of 

 the mountain that overlooks the springs. The rhyolites (No. 1, Captain 

 Button's catalogue) soon disappeared, and were succeeded by a dark, 

 coarse basaltic breccia. Masses of dark, fine-grained basalt, as much 

 as 3 or 4 feet in diameter, are mingled with the finer materials, which 

 are also basaltic, although of a great variety of colors and textures. I 

 did not attempt to reach the summit of the mountain on account of the 

 snow, but feel assured that the breccias and conglomerates constitute 

 the bulk of the mountain from this point up. In descending to the 

 north I soon reached a shelf-like space, the floor of which was composed 

 of the usual rhyolite. This is at a level corresponding pretty closely to 

 that of the other occurrences of similar rock about the rim of the valley. 



Between the rhyolite ledge and the springs the steep slopes are 

 clogged with fallen materials. By the yielding of the softer rocks be- 

 neath, large masses of the rhyolite become detached and gradually sink 

 into the deep amphitheaters beneath. 



About 500 feet below the outcrop of rhyolite a heavy mass of horn- 

 blende trachyte makes its appearance through the debris. Its relations 

 to other rocks cannot be ascertained. It weathers into round massive 

 forms above, but below breaks up into angular fragments. It is hard 

 and heavy and yellowish gray in color, and contains numerous large 

 • 2 H, PT II 



