GEOLOGICAL SUEVEY OF THE TEREITOEIES. 45 



very clear. I am inclined to think that the earlier force operated to 

 elevate the long continuous ranges of mountains, the nucleus of which 

 is the granitoid rocks, with the unchanged sedimentary beds upon the 

 sides and summits inclining at various angles. There was originally a 

 general trend to these mountain ranges that might have been called spe- 

 cific, perhaps, and\n the aggregate it is quite clear at the present time, 

 and is a little west of north. But when we come to study the minor 

 ridges, the unchanged rocks seem to incline in every direction and at 

 all angles from 1^ to 90°, and even sometimes past a vertical. Another 

 force, which has greatly influenced the form of the surface, and one 

 which, whether it operated synchronously or not, certainly acted with 

 full power at a subsequent period, concealing the metamorphic rocks and 

 the older sedimentary strata over large areas, and building up most of 

 the loftiest peaks. In the previous pages of this report, 1 have constantly 

 alluded to the exhibitions of the outflow of igneous matter at almost 

 every point of our journey ; but about the head-waters of the Missouri 

 and Yellowstone,! have estimated that at least three-fourths of the area 

 is covered with igneous rocks. Taking the valley of the Yellowstone 

 from its sources in the great water-shed to the mouth of Shield's Eiver, 

 an area one hundred and fifty miles from north to south, and fifty from 

 east to west, we find the evidences of volcanic action upon a tremendous 

 scale, and igneous rocks cover almost the entire area. Wherever the 

 metamorphic and sedimentary rocks are exposed in the vicinity of these 

 extensive outflows of igneous material, their history becomes much 

 complicated and the difficulties encountered by the geologist are greatly 

 increased. The valley of the Gallatin, like the valleys of all the streams 

 in Montana, is undoubtedly one of erosion originally, and was also the 

 bed of a lake. This lake basin extended down to the junction of the Three 

 Forks northward, and the modern deposits are found all along the base 

 of the mountains on either side of the valley up to the very sources of the 

 river, sometimes rising quite high on their sides. So great has been the 

 removal of sediment during and since the recession of the waters of the 

 lake, that it is not always easy to determine the entire thickness of the 

 original deposit. Kemnants are left, however, at different points, some- 

 times in the higher ranges of foot-hills, or in patches among the meta- 

 morphic rocks at considerable elevation on the divides between the 

 Gallatin, Madison, and Jefferson Forks. Areas of greater or less extent 

 occur 600 to 800 feet above the channels of the rivers, showing that the 

 waters must have been so high that only the more elevated summits were 

 above the surface. Opposite Fort Ellis are some high hills 600 to 800 

 feet above the valley below, composed of the well-known Pliocene marls, 

 sands, sandstones, and pudding-stones, horizontal for the most part, or 

 inclining at small angles. Among these beds are outflows of basalt in a 

 number of localities, but the disturbance of this group has been slight. 

 In most cases these deposits jut up against the sides of the mountains, 

 and when occurring in contact with the older rocks do not conform. 

 The group of hills opposite Fort Ellis extend down nearly to Flathead 

 Pass, and, having escaped erosion and removal for the most part, are left 

 as some proof of the original thickness of the lake deposit. Upon the 

 tops of the hills there is a considerable thickness of local drift, and scatv 

 tered thickly over the surface are rounded bowlders in great numbers 

 and variety. 



To study the older rocks to advantage, we must extend our examina- 

 tions to the numerous gorges, or canons, in the mountains, which, cutting 

 through the ux)heaved ridges at right angles, reveal more or less clearly 

 the order of the superi^osition of the strata. In Flathead Pass, Bridger 



