IIG GEOLOGICAL SURVEY OF THE TERRITORIES. 



stones form the center of the ritlge, the outer layers on either side dip- 

 l)ing in opposite directions, on tlie eastern side dipping northeast and 

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

 east. The exposures in the canon were very fine, but I found nothing 

 differing from what I have described in Spring Canon. 



On the 13th of July I started on a trip to Mystic Liike, 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 S])ring Canon 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 Canon 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. INIystic 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 banki^. 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 Spring Caiion Ridge 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 Cation 

 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, 500-00°. 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 vsome 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 00° 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 Caiion, 

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

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

 undoubtedly Carboniferous, containing Producttis lo)igifipinus, Spirifer 

 lineata, Hemipronites crene-stria, Productus scabricuhts, Zaphrentis, «&c. 

 The thickness of these beds is over25000 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 INIount Ellis, the extreme northern end 

 of the spur or ridge. The elevation of Mount Ellis is 8,419 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 probably contemporaneous with that of the Bridger Bange, 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 



