HOLMES.] THE EAST GALLATIN MOUNTAINS. 25 



from the eastern end of the spur just mentioned, looking: to the north 

 across the deep valley of Indian Creek. The position of the laccolite 

 wall will be understood by reference to the general view of the range 

 as shown in Plate IX. To the right we have the valley of Gardiner's 

 liiver, with Sepulchre Mountain and Emigrant Peak beyond. The 

 length of the wall shown in tlie drawing is about 3 miles, and the height 

 about 3,000 feet. The laccolite mass is well defined on the right, but 

 on the left is much obscured by accumulation of debris Indeed, the 

 valley of Indian Creek does not probably extend far enough to the left 

 to reach the western limit of the mass, if the arch should even be sym- 

 metrical. Along the lower part of the wall and about 800 feet above 

 the creek is a remarkably persistent wall of rock, that has a marked 

 vertical jointage, which I afterwards determined to be composed of Si- 

 lurian limestones. These limestones have a slight dip to the north, 

 which is not visible from my point of observation. Eesting upon the 

 apparently horizontal limestone floor as a base, is the laccolite, which 

 has, as can be clearly seen, separated the strata, and, by enormous ac- 

 cumulations of the molten material, arched the superposed strata over 

 its domed summit. The strata which overlie the trachj'^te are probably 

 mostly Carboniferous, as Carboniferous fossils occur at the right, but 

 little above the eastern edge of the laccolite. The width of the lacco- 

 lite from east to west is not less than 3 miles, and its thickness in the 

 middle portion will not fall far short of 1,000 feet. There are a number 

 of lines running across the steep face of the laccolite that are appar- 

 ently included fragments of the sedimentary rocks, and probably mark 

 the junction of separate intrusions. 



In the bed of Indian Creek, beneath the base of the eastern edge of 

 the laccolite, the granites appear. A low, wooded hill on the north 

 side, at the exit of the creek from the mountain, is underlaid by these 

 rocks. Between the hill and the laccolite wall is a depression through 

 which an Indian trail passes. It appears to have been the trail used 

 by the band of Bannocks who made a raid through the Park in August. 

 It follows up Indian Creek and crosses the mountains to the head of 

 the Gallatin Eiver. 



In ascending the cliffs to the base of the laccolite, I found first at the 

 base a covered space of some 300 feet, with outcrops toward the top, 

 and just beneath the great limestone wall of greenish-gray micaceous 

 shales. These are probably the lowest Silurian rocks of this region and 

 lie directly upon the granite. I found the great wall to consist of up- 

 wards of 300 feet of dark-gray limestones, which, although apparently 

 at one time rather finely bedded, are now exceedingly compact. The 

 vertical lines of weathering which have such a remarkable appearance 

 at a distance prove to be owing to a rudely columnar structure, super- 

 induced either by metamorphism or by some conditions of the weath- 

 ering. I found no fossils, although the planes of bedding show many 

 markings that are probably due to the presence of animal life at the 

 time of their formation. 



The laccolite is composed of hornblendic trachyte, in every respect like 

 that on the south side. It is 400 to 500 feet thick at the point visited, 

 but at the highest part of the arch is not less than 1,000 feet. The 

 arched strata that overlie the laccolite, are, so far as the fallen masses 

 indicate, yellowish and grayish limestones and siliceous limestones, 

 without fossils. They comprise a thickness of from 500 to 800 feet. 

 (See general section, Plate XIV). 



Passing around the east face of the laccolite spur, I found, just be- 



