GEOLOGICAL SURVEY OF THE TERRITORIES. 19 



been studied so minutely east of the Mississippi. The evidence, hou^- 

 ever, so far as I have been able to i^/rocure it as yet, is against any AveH- 

 defiued lines of demarkation ; that the fossils which have been employed 

 by paleontologists at the East to characterize certain beds pass from 

 one to the other in these western groups, so that no weildefiued line 

 can be drawn in most cases. 



As far back as 1857, while the writer was connected with the explor- 

 ing expedition of Lieutenant Warren, United States Topographical 

 Engineers, he obtained, from a series of reddish calcareous sandstones, a 

 group of fossils, which Mr. Meek at once identified as belonging to the 

 horizon of the Potsdam sandstone of New York. The key having been 

 once secured to the age, it was not difficult to extend the area of this group 

 farther west and north and south on a geological map, and one was pre- 

 pared by me and published in connection with my report for 1801), which 

 indicated the existence of this division all along the margins of the east- 

 ern Ivocky Mountain ranges. But it was not known until the present 

 season that still higher members of this group existed in the far West. 

 While the Snake Eiver division was passing up the Malade Valley, 

 Professor Bradley discovered masses of limestone filled v/ith fragments 

 of trilobites that indicated the existence of the Quebec group. The 

 proof once made known from some favored locality, it was not difficult to 

 extend the geographical area over the greater part, or perhaps all the 

 area, of the Northwest. x\t any rate, the party under my direction found 

 this group well developed over the greater portion of Montana. Along 

 the Gallatin Eiver, near the Three Forks, and below, the Silurian beds 

 reach an aggregate thickness of 1,600 to 2,000 feet, and most probably 

 include the entire Potsdam group. Future explorations in localities 

 where the conditions are favorable, may reveal the existence of other 

 subdivisions of the Silurian, Devonian, or Carboniferous. The discovery 

 of the well-known Silurian coral, Hahjsites cafenqwra, iu the summer of 

 1871, near the sources of Ogden Creek, points to the existence of the 

 Niagara group. 



In the spring of 1859, a large expedition was organized under the 

 War Department for the i)urpose of exploring the sources of the Mis- 

 soiiri and Yellowstone Elvers, and placed under the direction of Captain 

 Wiliiam F. Eayuolds, Topographical Engineers. To this expedition 

 the writer was attached as geologist. The party started across the 

 country from Fort Pierre, on the Missouri Eiver, iu the spring of 1859, 

 passed along the north side of the Black Hills to the valley of the Yel- 

 lowstone, and then up that valley to the mouth of the Big Horn Eiver, 

 and then up the valley of the Big Horn to the Big Horn Mountains 

 southward to Deer Creek, a tributary of the North Platte about one 

 hundred miles above Fort Laramie, where they spent the winter. In 

 the spring, the party passed up the North Platte, by way of the Eed 

 Buttes, to" the valley of Wind Eiver, ascended that stream to its source, 

 and crossed theW^ind Eiver Mountains over Union Pass into the valley 

 of Snake Eiver, crossed that stream near Jackson's Hole, passed up the 

 valley northward across the sources of the little streams running into 

 Henry's Fork on the east side, and entered the valley of the Madison 

 through Low or Eaynolds's Pass. Captain Eaynolds's report, accompa- 

 nied by an excellent map, was published by Congress in 1808, and the 

 report of the geologist, accompanied by a geological map in colors, was 

 published iu 1809. As these reports are now out of print, I take the 

 liberty of making such extracts gs will be of interest in this connection. 

 The portion recording my observations of the geology about the Wind 

 Eiver Mountains, Snake Eiver Valley, Tetons, &c., is comprised iuthe 



