116 GEOLOGICAL SURVEY OF THE TERRITORIES. 



Stones foriM the center of the ridge, the outer layers on either side dip- 

 ping in opposite directions, on the eastern side dipping northeast and 

 those on the western side southwest. The general strike is south 30^ 

 east. The exposures in the caiion were very iine, but I found nothing 

 differing from what I have described in Spring Caiion. 



On the 13th of July I started on a trip to Mystic Lake, about twelve 

 miles nearly south from Fort Ellis. After a pleasant ride over the 

 grassy plain that slopes gently from the mountain's edge we began to 

 ascend and soon found ourselves entangled in a mass of dead and 

 fallen timber. After considerable trouble we reached the summit of the 

 hill to find that we had to descend again on the opposite side. The hills 

 are so covered with vegetation and debris that the character of its rocks 

 cannot be made out certainly. High up on the sides, however, there 

 are small exposures of limestones and sandstones, and it is probable that 

 the valley is underlaid by Cretaceous formations. What their exact 

 relation is to the ridge running south from Spring Caiion it is rather 

 difficult to determine, though it is probable that the trail runs through 

 a synclinal valley on one side of which lies the Spring Caiion Eidge. 



The trail leads us now through pine-forests and anon across beautiful 

 little valleys, each a garden of wild flowers. At last, after crossing 

 several ridges, we reach the lake. Mystic Lake is the head of Bosemau. 

 Creek, one of the branches of the East Gallatin Eiver. Near it, on a 

 level fully one hundred feet higher, are two exquisitely beautiful lakes, 

 whose beauty is half hid by the trees fringing their banks. One of them 

 we named Emerald Lake, from the deep-green tint of its waters. 



The valley in which these lakes are situated is synclinal, one side being 

 the continuation of the Sjiriug Caiion Eidge and the Other a spur run- 

 ning south from Mount Ellis. Opposite the lake, to the southeast, there 

 is a volcanic range, at the base of which we find the Spring Canon 

 layers, having a general dip to the southwest. The other side of the 

 synclinal cuts obliquely across the lower end of Mystic Lake, the strata 

 dipping north 40° east; angle, 500-60°. At present the lake is about 

 three-fourths of a mile in length and about one-fourth of a mile wide. It 

 once extended farther up the valley and lay in a saucer-like depression. 

 From the gradual elevation of the valley or some other cause, as the 

 draining of the lake by the erosion caused by its outlet, at present it 

 occupies the lower end of the valley, lying on the edges of the Cretace- 

 ous, Jurassic, and Carboniferous strata. 



The course of the stream of which Mystic Lake is an expansion is 

 about south 20° west, making an angle of about 60° with the strike of 

 the strata. On the western shore of the lake there are exposures of 

 Jurassic sandstones and limestones precisely like those of Spring Canon, 

 and in which occur Ostrea and Camptonectes. Below the Jurassic come 

 immense beds of quartzites and limestones, the upper layers of which are 

 undoubtedly Carboniferous, containing Productus longisjnnufi, Spirifer 

 lineata, Hemipronites crenestria^ Productus scahriculus, Zaphrentis, &c. 

 The thickness of these beds is over2,000 feet, and the lower strata should 

 probably be referred to a lower geological horizon than the Carboniferous. 

 As we go south the strata turn more and more toward the west. As 

 we go toward the north we find Mount Ellis, the extreme northern end 

 of the spur or ridge. The elevation of Mount Ellis is 8,410 feet above 

 the sea. It is composed mainly of Carboniferous limestones, while to 

 the west and at the base are gneissic rocks. The elevation of this ridge 

 was i)robably contemporaneous with that of the Bridger Eauge, as its 

 formation seems to be similar. As we follow the ridge southward it 

 curves until the trend is almost east and west. It probably once formed 



