24 EEPORT UNITED STATES GEOLOGICAL SUEVEY. 



smooth. The limestone and quartzite strata, being rather thinly bedded, 

 break down in small fragments which form into slides and fill np the un- 

 even places. The drainage of the peak appears to be entirely tributary 

 to the Gardiner River. Extending from the summit to the north, along 

 the right side of Indian Creek, is a high, narrow ridge, which lollows 

 the course of Indian Creek as it turns to the east, and terminates in a 

 high point, which in the drawing (Plate IX) forms the second ridge, 

 the first being the eastern spur of Mount Holmes or the Trilobite Eidge, 



The geology of this ridge, and of the crest of the divide which ex- 

 tends along the left bank of Indian Creek, I could only determine in a 

 general way. The strata are chiefly Silurian, with probably a moderate 

 thickness of Carboniferous rocks on the higher summits, and with very 

 considerable bodies of intruded trachyte, which in its manner of occur- 

 rence resembles very closely the hornblendic trachytes of Southwest 

 Colorado. The highest point on the northern spur of Mount Holmes is 

 apparently formed of trachyte, which belongs to one of these intruded 

 masses, and which has turned the Paleozoic strata up at pretty high 

 angles around it. 



South from Mount Holmes some 3 miles there is an outstanding mount- 

 ain mass, which connects with the Gallatin Eange just west of the 

 former mountain. It lies between the headwaters of Obsidian Creek 

 on the east and a northern branch of the Madison on the west. After 

 descending from Mount Holmes I made an attempt to reach this point, 

 but failed for want of time. The rocks so far as I went are granites. 

 It is probable, however, that there are Silurian rocks on the higher points. 

 Beyond this point there is a gradual descent to the Madison Plateau, 

 of which such a fine section is seen in the third cafion. The outline view 

 of Mount Holmes given in Plate VIII was made from the edge of Madi- 

 son Plateau, opposite the mouth of Fire Hole Fork. 



On the morning of the 9th I left camp at an early hour, and passed 

 around the timbered base of Trilobite Spur to the north. The granite 

 bench continues along the base of this spur, and is last seen on the 

 north bank of the main Indian Creek. The steep walls that rise from 

 the granite bench are formed of Silurian quartzites and limestone, with 

 intercalated masses of hornblende trachyte. On the south side of In- 

 dian Creek, at the eastern end of the northern spur of Mount Holmes, 

 the granitic rocks rise to the height of some 600 feet above the creek, 

 and seem to have a rather uneven upper surface. They have some in- 

 dications of bedding, with a sharp dip to the north. The granite is 

 not so massive and compact as that shown at the south base of Mount 

 Holmes, and has in places the appearance of a flinty quartzite in which 

 are imbedded occasional large crystals of light-colored feldspar. At 

 a short distance these rocks look like a metamorphosed conglomerate. 

 Eesting upon the rather uneven surface of the granitic rocks are very 

 bulky sheets of hornblendic trachyte, which is a beautiful rock of a 

 light-gray color (No. 18, Captain Button's catalogue). It is exceed- 

 ingly compact, and the fallen masses break into large angular frag- 

 ments with smoothly fractured faces. From the outer edge of this 

 spur we have a fine view of the magnificent wall that rises on the north 

 side of Indian Creek, the section exposed being one of unusual interest. 

 We have a special section of a "laccolite," the structure of which is 

 apparently identical with that of the laccolites of Colorado and Utah, 

 which have been so well described in Mr. Gilbert's report on the 

 Henry Mountains. There will be no better way of presenting this 

 than by the accompanying sketch (Plate XIII). This sketch was made 



