24 GEOLOGICAL SURVEY OF THE TERKITORIES. 



from which these streams take their rise. As we descend the Madison 

 we find that the valley seems to i)ass along a sort of anticlinal axis, and 

 on each side loftj^, nearly vertical walls of trachyte, arranged in thick 

 layers. The lower portion a])pears to yield quite readily to atmospheric 

 agencies, owing to the ferrngiuous matter contained, which renders it a 

 loosely aggregated mass of crystals of feldspar. As we ascend upward 

 the rocks become more compact, and the upper layers are a cellular tra- 

 chyte. In some places the upper compact beds assume a columnar 

 structure, breaking into the form of vertical columns; these break in 

 pieces and cover the sides of tlie hills with masses of rock. Lower 

 down on the Madison we find layers of the red feldspar, which present 

 the appearance of stratified beds like the Azoic, rocks, with an inclina- 

 tion in the same direction with the overlying basaltic rocks. There are 

 numerous seams of white quartz, also trap, running across the country 

 in every direction, many of which indicate the presence of gold-bearing 

 rock. The suujmits, or crests, of the high mountains are ragged, not 

 from erosions since upheaval, but owing to the manner of the upheaval. 

 Each peak assumes, to a certain extent, the form of an independent uplift, 

 with layers of rock inclining around from every side; and yet it is by a 

 series of these peaks connected together, more or less, that a mountain 

 range is formed. Wherever these peaks or groups of peaks are separated 

 a short distance, a low point is made in the range, which gives passage to 

 streams. Very many of these low passes have no streams issuingfrom them 

 at this time. The Madison forms a caiion by cutting through one of these 

 lofty ridges at the upper portion of the Burnt Hole, and a second one at 

 the lower end of the same valley. Still below the feldspar beds and 

 near the junction of the Three Forks of the Missouri we have beds of 

 exceedingly slaty character, inclining at angles of 31° and passing down 

 into the granitoid rocks below. 



" In' the valleys of these streams is a series of marls and marly sands 

 and conglomerates, precisely like the upper beds of the White Eiver 

 Tertiary. These marls are mostly of a flesh-color, sometimes assuming 

 the texture of a quartzose sandstone. Its greatest thickness in this 

 region is about 200 feet, and not conforming to the Carboniferous rocks 

 beneath, but inclining in the same direction about 8°. 



" The Carboniferous rocks are largely developed in this region, and 

 incline at very large angles from the mountains. The lower part of 

 these limestones have been so affected by heat that the stratification has 

 been very nearly obliterated, and presents a very rough appearance. 

 Above this is a bed which is undoubtedly Carl3oniferous limestone 

 changed, but which now very much resembles basalt, but contains more 

 arenaceous matter, and appears to havehad the stratification but partially 

 changed. From the Three Forks these limestones extend westward or 

 southwestward about twenty to twenty-five miles, and then continued 

 northward toward the gate of the mountains along the Missouri. They 

 also extend to the northwest to a range of mountains, in which is the 

 Blackt'oot Pass of Lewis and Clarke. 



" July 3. — Visited the plateau, mentioned b^^ Lewis and Clarke, be- 

 tween the mouths of the Gallatin and Sladison. It is a long flat ridge 

 of limestone, representing the portion of the inclined rocks which form 

 the left side of the caiion below. 



" The rocks on that side incline 24°, continuing far on the distant hills. 

 The base of this small ridge is a bluish cherty limestone, sometimes yel- 

 lowish, very compact or hard, breaking into fragments just like the 

 Carboniferous limestones before seen. Dip, 33°. This bed corresponds 

 with a portion of the right side of the caiion next to the water. There are 



