28 GEOLOGICAL SURVEY OF THE TERRITORIES. 



posed of gneissic or granitoid rocks of every possible texture. These 

 constitute the nucleus of nearly all the principal mountain-ranges. 



From the above brief outline it will be seen that two divisions of the 

 geological scale, Triassic and Devonian, are not represented in Montana, 

 so far as we have the evidence up to the present time. We may state, 

 however, in this connection, that we do not deny the existence of these 

 formations in this portion of the West, only that the rocks have yielded 

 us no such information as yet. When we reflect that, in countries where 

 the geological formations have been studied for nearly a century by the 

 ablest minds under the most favorable circumstances, they are yielding 

 up new and startling facts every year, we cannot hope in a preliminary 

 survey of so vast an area to exhaust the discoveries. 



In the great area which comprises what we term the Eocky Mountain 

 region, the groups of strata mentioned above appear and disappear 

 iu a strange manner at times, thus rendering their study more difficult 

 and laborious than one might suppose at first sight. 



Sometimes the Carboniferous limestones, with their characteristic 

 fossils, appear to rest on the gneissic beds below ; then again there will 

 be 1,500 to 2,000 feet of Silurian strata intervening. At one locality a 

 certain group of metamorphic beds will occur, and at another, a series 

 quite distinct in texture. In one locality the red beds will be well de- 

 veloped, reminding one of the possible existence of the Triassic ; in 

 another locality, not far distant, no trace of these can be found. The 

 same may be said of all the groups of strata. One peculiarity of the 

 coal-strata consists in their dark somber color, which I have observed 

 nowhere else except in this region. 



In order that all the details of the local geology may be more clearly 

 set forth, I have continued the same plan in this report which was 

 adopted in the previous ones, of describing the geological features of 

 the country as observed along the routes traveled. 



The i^arty under my direction left Fort Ellis for the Yellowstone 

 Valley July 20. Much scientific work had been done previous to that time, 

 but during the present season the deep snows on the mountain-ranges 

 would have prevented successful exi^loration in Montana before the 10th 

 of July. The previous winter had been one of the severest ever known 

 in the Territory, and the snow remained until late in the summer. Dur- 

 ing the melting of these winter-snows the streams are so high that 

 traveling becomes very difltictilt. The season of exi)loration is compara- 

 tively short. 



Our entire outfit having been completed at Bozeman and Fort Ellis, 

 we passed up the grassy valley of Spring Creek, by way of Bozeman 

 Pass, to the summit of the divide. Just opposite Fort ElliS; on the east 

 side of the creek, is a ridge of gray and grayish-white marly sands and 

 sandstones, with nearly horizontal strata jutting mj against the older 

 rocks. This ridge presents a fine example of the lake-deposits which 

 are so common in the valleys all over the West. It has evidently es- 

 caped the erosion which has swept away the greater portiougpf these 

 deposits, and this ridge may now be regarded as a remnant — a monu- 

 ment left to indicate the approximate thickness of the original beds. 

 The greatest thickness of these beds in this valley was probably about 

 1,500 feet. The summit of this ridge is 800 feet above the valley, and 

 its greatest thickness could not have been less than 1,000 feet. The 

 evidence is clear that all these valleys were once filled with water, so 

 that only the summits of the loftier portions were exposed ; that prob- 

 ably, during what geologists term the Quarternary period, these waters 

 gradually subsided, sweeping downward, to the lowlands near the main 



