18 EEPOKT UNITED STATES GEOLOGICAL SURVEY. 



crystals of hornblende. In general appearance it is much like the horn- 

 blende trachytes of southwestern Colorado. Similar rocks occur largely 

 in the Mesozoic and Paleozoic rocks in the Gallatin Eange, 10 miles to 

 the west of this point. It is quite probable that this mass is part of a 

 sheet interbedded with Cretaceous rocks, which from its greater hard- 

 ness has resisted the erosive forces. 



On the day following I rode around Sepulchre Mountain to the north, 

 crossing, for the first mile, smooth, grass-covered slopes, over which are 

 scattered numerous fragments of hot-spring limestone and other rocks. 

 A little beyond the first rivulet there is a low hill, capped with basaltic 

 breccia, and beyond this on the opposite side of a second stream another 

 hill or butte, much larger than the first, which is also formed of the 

 basaltic breccia. In the depression between these hills there are a num- 

 ber of small lakes or ponds, which seem to occupy depressions formed 

 by masses of conglomerate that have slid down the slopes and inter- 

 cepted the streams. It appears that the whole slope from the base of 

 the massive walls of conglomerates above down to the wagon road, and 

 even in places to the river, is covered with masses of the breccias and 

 conglomerates that have come down from the summit of the mountain. 



Farther around to the north, directly under the high walls of the main 

 summit, this degradation by land slides has taken place on a grand 

 scale, and the same agencies are still actively at work. Many of the 

 larger spurs and outstanding masses are nothing more than sinking 

 fragments of the conglomerates. Great terrace-like masses of conglom- 

 erates have detached themselves from the main mass above, and still 

 remain but little depressed, but are separated from the present mass by 

 deep crevices. Some of these masses have progressed still further and 

 are beginning to break up into blocks. The forest trees that cover these 

 masses present a very extraordinary appearance, as they are tipped 

 about at all angles. To one who did not understand the causes of these 

 movements the region would present the appearance of having been 

 subject to violent volcanic action, or to the movements of an earthquake. 

 The occurrence of these gigantic slides is easily accounted for. The 

 heavy, coarsely bedded, and rather compact conglomerates of the upper 

 part of the mountain are everywhere underlaid by a series of fine-grained 

 uncompacted material, chiefly marls, sand, and shales. In moist seasons 

 this soft rock gives way, and rapid degradation is the direct conse- 

 quence. Much of the drift material in the surrounding valley is derived 

 from the disintegrated conglomerates. 



Along the lower slope of the mountain, on the north side, there is a 

 band of light-colored marls that seems to underlie the conglomerates and 

 to rest unconformably upon the Cretaceous strata beneath. In the great 

 northern spur, that extends down to the river at the mouth of Cache 

 Creek, strata, apparently corresponding to the marls, lie beneath the 

 massive conglomerates that form the crest of the ridge. These strata 

 are light in color but generally coarse grained, and are composed of a 

 variety of materials that comprise all kinds of rocks from white vol- 

 canic tuffs to masses of feldspathic granite. I did not have time to 

 observe these strata carefully, and hence am in much doubt in regard 

 to them. They are apparently quite modern, and it is not improbable 

 that they are Pliocene lake deposits, as the conglomerates, which overlie 

 them, are not probably later than late Pliocene, and these beds were 

 -evidently deposited in a body of water that existed at the beginning of 

 the conglomerate building period. On the north side of the point of the 

 cspur mentioned above a mass of hornblendic trachyte occurs, very sim- 

 ilar in appearance to that found above the springs. It is not probable 



