50 GEOLOGICAL SUEVEY OF THE TEEEITOEIES. 



(Eocene.) Interstratified with the beds of this group, are layers of basalt 

 of irregular thickness, some of which is so compact and homogene- 

 ous in structure that it must have cooled under much pressure, 

 and perhaps never reached the surface until exposed by erosion 

 or the elevarion of the mountain ranges. Then in the valleys of the 

 streams, some with flowing water, others dry, yet all deep and apparently 

 at one time the channels of large bodies of water, are great quantities 

 of the local drift and debris, concealing the underlying basis rocks so as 

 to perplex the geologist, and yet an active search will show that along the 

 banks of the stream, a fewfeet in thickness of some one of the formations 

 of the district will be exposed. It may be the oldest ; it may be the latest ; 

 we may find an outcrop of massive granites, of stratified granitoid 

 rocks. Carboniferous limestones, or the latest Pliocene marl group ; the 

 youngest rocks may cover the loftiest ridges, and vice versa. The Plio- 

 cene marls do not unfrequently occur in contact with the massive un- 

 stratified granites on the summit of the mountains, so that we may step 

 within a few paces from the youngest rocks known in the West to the 

 very oldest. The beautiful, regular curves and flexures in the strata, 

 -which continue so systematically over long-extended areas in Pennsyl- 

 vania and along the Atlantic border, are wanting in the Eocky Mount- 

 ains- Local curves of remarkable beauty occur in the strata, from 

 time to time, as we shall attempt to show by figures in the final report. 

 Altitude, therefore, gives no clew to the age of rocks. I have also given 

 the angle of inclination of the strata from time to time in my reports. 

 In regard to the more eastern ranges of the Eocky Mountains, the dip 

 and trend are terms possessing some force and meaning, but in the vol- 

 canic regions of the Yellowstone and Missouri Elvers such observations 

 seem to be of little value. There is no doubt that, when the whole 

 country has been carefully mapped and the geology worked out in detail, 

 a system will be found in the results of the action of the internal forces 

 that gave to the surface its present form. So in regard to the position 

 of the strata, altitude gives no clew ; the oldest, to the Cretaceous inclu- 

 sive, in the lowest valley, on the summit of the highest range, may be 

 horizontal or incline at any angle. The Carboniferous limestones on the 

 divide between Trail Creek and a little branch flowing into the 

 Yellowstone to the north are vertical, or nearly so, or have been lifted 

 up in broad areas to the summit of the divide, so as to be nearly or 

 quite horizontal, while all around it bend down the same limestones, 

 like the leaves of a table, at angles of 60° to 80°, and in a few in- 

 stances inclining past a vertical. In the valley of the Yellowstone, 

 these same limestones will be found horizontal, while upon the sum- 

 mits of the mountains, 3,000 feet above the valley, within a few miles, 

 they incline at a very moderate angle. These facts seem to show the 

 importance of having the topography of the country worked out with 

 great care in connection with the geology, in order that the multiplicity 

 of detail may be clearly expressed. 



From the summit of the divide down to the ravine of Trail Creek, we 

 can look to the eastward, into the beautiful valley of the Yellowstone Eiver. 

 On the south side is the high range of mountains, at first composed of 

 sedimentary rocks, with their jagged summits rising up 1,200 feet 

 above the valley, and after passing the divide, this range flexes around 

 to the south, extends up on the west side of the Yellowstone, forming, 

 the water-shed between the sources of the Gallatin and Madison Forks. 

 After passing the head of Trail Creek, this range is composed almost 

 entirely of igneous rocks, so far as they are revealed to the eye. There 

 is reason to believe, however, that underneath this vast mass of basalt 



