164 



GEOLOGICAL SURVEY OF THE TERRITORIES. 



the bed of a lake, wliich was probably one arm of the lake to which I 



have referred before as coveriDg the country 

 about the Three Forks of the Missouri, aud 

 extending also into the valleys of the Gallatiu 

 t^ and Jeiierson Elvers. Since the subsidence of 

 I the waters the gradual elevation of the country 

 © at the head of the river has caused it to carve 

 g out of these deposits a beautiful set of terraces, 

 g the most perfect I have ever seen. At the 

 I head of the valley there are four of them, all 

 ^ well defined. Some are capped with basalt 

 ^ and trachyte, which is columnar above and 

 •3 laminated below. OppositeVirginia City there 

 I are two, the first of which, on the west side of 

 t*" the river, is 53 feet in height and on the east- 

 ^" ern side is w-anting. The top of the sec- 

 g oud terrace is 243.5 feet above the level of 

 i the river on the western side and 149 feet 

 I on the eastern side. These terraces are but 

 5 the remnants of what once filled up the entire 

 '^^ valley, and are composed above of very soft 



1 sandstones, containing fine grains of mica. 

 g These sandstones pass down into conglomer- 

 ^ ates. The beds are all calcareous and nearly 

 Q horizontal in position. On the western side 

 B of the Madison Valley we find granites out- 



2 cropping, upon which are limestones probably 

 I from the Silurian upward. The Carboniferous 

 I beds, at any rate, are present in considerable 

 J thickness. The underlying rocks in the valley, 



upon which the lake-deposits rest, I take to be 

 fl granites. The accompanying illustration (Fig. 



1 42) will show what I conceive to be the struc- 

 S ture of the country. It represents a section 

 M" across the country in a westerly direction from 



the east side of the Gallatin Eiver to the west 



1 side of the Madison, The dotted lines repre- 

 (5 sent the rehition of the inverted beds, extend- 

 <{ ing along the entire Madison Range to the 

 ;: beds resting on the granites on the west side 

 I of the Madison. The force which inverted the 

 fl beds of the Madison range was probably ob- 

 I lique in its direction. The amount of erosion 

 I since the upheaval must have been enormous. 

 jgT I shall refer to this section again when speak- 

 - ing of the Gallatin Canon. 

 I When I visited Jackass Creek with Mr. 

 ^ 'Holmes and one of our packers, we left the 



.1 main party on the west side of the river, on 

 I which side they intended to proceed down 

 ^. stream some distance before crossing, while 

 ^ we were to go down on the east side skirting 

 the lower canon as closely as possible, and 

 after visiting the Cherry Creek mines to join 

 them as soon as we could overtake them. 

 We left Jackass Creek on the morning of the 8th and proceeded to the river, 



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