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GEOLOGICAL SURVEY OF THE TERRITORIES. 



the bed 



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

 have referred before as covering the country 

 about the Three Forks of the Missouri, and 

 extending also into the valleys of the Gallatin 

 and Jefierson Eivers. Since the subsidence oi 

 the waters the gradual elevation of the country 

 at the head of the river has caused it to carve 

 out of these deposits a beautiful set of terraces, 

 the most perfect I have ever seen. At the 

 head of the valley there are four of thein, all 

 well defined. Some are capped with basalt 

 and trachyte, which is columnar above and 

 laminated below. Opposite Virginia City there 

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

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

 ern side is v\'anting. The top of the sec- 

 ond terrace is 24:3.5 feet above the level of 

 the river on the western side and 149 feet 

 on the eastern side. These terraces are but 

 the remnants of wliat once filled up the entire 

 valley, and are composed above of very soft 

 sandstones, containing fine grains of mica. 

 These sandstones pass down into conglomer- 

 ates. The beds are all calcareous and nearly 

 horizontal in position. On the western side 

 of the Madison Valley we fiud granites out- 

 cropping, upon which are limestones probably 

 from the Silurian upward. The Carboniferous 

 beds, at any rate, are present in considerable 

 thickness. The underlying rocks in the valley, 

 upon which the lake-de})osits rest, I take to be 

 granites. The acconipanying illustration (Fig. 

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

 ture of the country. It represents a section 

 across the country in a westerly directiou from 

 the east side of the Gallatin lliver to the west 

 side of the Madison, The dotted lines repre- 

 sent the relation of the inverted beds, extend- 

 ing along the entire Madison Range to the 

 beds resting on the granites on the west side 

 of the Madison. The force which inverted the 

 beds of the Madison range was probably ob- 

 lique in its direction. The amount of erosion 

 since the upheaval must have been enormous. 

 I shall refer to this section again when speak- 

 ing of the Gallatin Cahou. 



When I visited Jackass Creek with Mr. 

 Holmes and one of our packers, we left the 

 uuun party on the west side of the river, on 

 which side they intended to proceed down 

 stream souie distance before 



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^ we were to go down on the 



(;rossing, while 



east side skirting 

 the lower caSon 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, 



