132 GEOLOGICAL SURVEY OF THE TERRITORIES. 



Leaving our camp at Flat Mountain, we ascended tlie liigh hills, 

 among the fallen timber, taking a course about southeast, passed over 

 the divide, and at night found ourselves on the head- waters of Snake 

 Eiver. The rocks, as usual, were trachytic basalt, for the most part ; but 

 in ascending the divide from the Yellowstone Lake, we find Carboniferous 

 limestones, with the accompanying clays, in one locality. Examples of 

 the exfoliation of the igneous rocks are very common. 



Between Flat Mountain and the Yellowstone Eange the divide is very 

 low. The sources of some of the branches of Snake Eiver extend up 

 within two miles of the lake, and the elevation is not more than 400 feet 

 above the lake level. This is what has been hitherto understood as 

 '' Two Ocean Pass.'' The separation of the drainage between the 

 Yellowstone Basin and Snake Eiver is complete. The valley of Snake 

 Eiver is very j)leasantly diversified with meadow -like openings and 

 dense forests of pines. Some of these glades are two to four miles long 

 and one to two miles wide. This transition from forest to meadow con- 

 tinues all along the river and its branches, from their sources to the 

 junction with the Columbia. 



From our camp on Snake Eiver, we traveled north of east to the shores 

 of the lake. The broad lowlands are most pleasantly diversified with 

 groves of i)ines and fine grassy meadows, and numbers of streams, some 

 of which were of considerable size, flowed from the mountains into the 

 lake. One of these creeks was 75 feet wide and 2 feet deep. All 

 the rocks we met with were basalt and basaltic breccia. The Yellowstone 

 Eange, so far as I could examine it, was composed of breccia, though it 

 undoubtedly contains a nucleus of trachyte j for the masses of it, which 

 could not have been transported far, were scattered over the surface. 

 We crossed the marshy valley of the Upper Yellowstone, which is about 

 three miles wide, and pitched our tents upon a sort of terrace on the 

 east side of the southeast arm, 80 feet above the water-level of the lake. 

 From this point we made a small side trip to the source of the Upper Yel- 

 lowstone, and thence to the sources of the Snake Eiver. The entire region 

 is one of great interest. On the morning of August 12, 1 started up the 

 valley of the Upper Yellowstone, accompanied by Messrs. Doane and 

 Schonboru, for the purpose of making a careful geological as well as 

 topographical survey of the district bordering the great divide. Five 

 streams of water flow into the Upper Yellowstone from the mountains on 

 either side of the head of the valley, and at this season of the year the veg- 

 etation is fresh, green, and most abundant. It would be difficult to find a 

 valley in the West that presents as fine a picture to the eye. On either 

 side, the valley, which is about three miles wide, is walled in by dark, som- 

 ber rocks of volcanic origin, which have been weathered into remarkable 

 architectural forms. Looking up the valley from some high point, one 

 could almost imagine that he was in the presence of the ruins of some 

 gigantic city, so much like old castles, cathedrals of every age and clime, 

 do these rocks appear ; add to this, the singular vertical furrows which 

 are cut deep into the sides and render more striking their antiquated ap- 

 pearance. At the base of the wall-like ridges of the valley, immense 

 masses of volcanic breccia have fallen down from the mountain-tops, in 

 many instances crushing down the pines along their path. About fifteen 

 miles above the lake the valley terminates abruptly, the mountains 

 rising like walls, and shutting off the country beyond. The river here 

 separates into three main branches, with here and there smaller ones, 

 which bring the aggregated waters of the melted snows from the sum- 

 mits of their bare volcanic peaks. Just at the head of the valley there 

 is a little lake, but not more than one or two hundred yards in width. 



