GEOLOGICAL SUEVEY OF THE TERRITOEIES. 27 



in Montana wbicli have come witliiu the limit of our explorations. The 

 town of Bozeman is located near the upj)er or south end, and Fort Ellis 

 lies about three miles to the southeast, under the shadow of the mount- 

 ain-ranges that form the water-shed between the Missouri and the 

 Yellowstone. On the east side of the valley is the Gallatin liange of 

 mountains, which gives origin to numerous branches of the Gallatin 

 Eiver on the west side, and many branches of Shield's River on the east 

 side. Ou the north side of the valley is a series of broken ranges, which 

 give origin to immbers of branches of the East and West Gallatin Rivers. 

 A ridge or low divide extends down between the Gallatin and Madison 

 Valleys, and entirely disappears before reaching the junction of the 

 Three Forks. This valley is about forty miles in length from north to 

 south, and five to fifteen miles in width. This valley may be regarded 

 as typical of the general character of the surface of Montana, as well as 

 parts of the adjacent Territory which were examined by the survey. Two 

 general divisions might be made of the entire surface, mountaiu and val- 

 ley. The valleys, and the portions which are open to settlement are, at 

 the present time, occapied to a greater or less extent by thriving farmers, 

 with here and there i>rosperous villages. They are for the most part old 

 lake-basins, geologically of comparatively modern date. Along nearly 

 all. the more important rivers, from their sources to their entrance upon 

 the plains, there is a chain of these valleys, varying in length from a mile 

 to fifty or sixty miles, and connected by a cleft or gorge in the mountains, 

 through which the river has worn its way. In all these valleys there is 

 a greater or less thickness of deposits, very similar in character, of a 

 liglit-gray or cream color, and composed mostly of clay, lime, and silica 

 in various proportions. Very few fossils have been found in these de- 

 posits about the sources of the great rivers, but it is most probable that 

 the deposits are of the same age in Western Idaho, Oregon, and Califor- 

 nia which have yielded large quantities of vertebrate remains. In the 

 summer of 1871 I discovered in these lake beds species of Ancliither'mm 

 in the head of the Jefferson Fork, and with it were associated fresh-water 

 and land shells. But these beds yield the most beautiful forms of silici- 

 fied wood that are found in any part of the continent. It is sometimes 

 called opalized wood, and it was doubtless formed in connection with hot 

 springs. It is most ])robable that during the Pliocene period hot springs 

 prevailed to a greater or less extent all over the western portion of our 

 continent, and their action may serve to account for many i^roblems 

 which now seem obscure. This deposit varies fix)m a few" feet in thick- 

 ness to 1,000 or 1,500 feet, and is usually nearly horizontal, resting 

 unconform ably upon the older rocks. ISTot infrequently these beds incline 

 5°, indicating slight changes in the general level of the surface since 

 their deposition. 



Then we have a vast thickness of what may be called coal-strata in 

 the W^est, the age of which seems obscure. They contain the great and 

 valuable deposits of coal in the West, and are thus of the utmost im- 

 portance in an economical point of view. This group varies in thickness 

 from 1,000 to 5,000 feet, and in some parts of the country may reach the 

 thickness of 10,000 feet. The evidence at the present time points to the 

 conclusion that the lower j^ortions of this group are Cretaceous, passing 

 up by gradual transition into the Tertiary, and that the greater portion 

 may be regarded ak of the age of the later period. Then follow in de- 

 scending order the Cretaceous, Jurassic, Carboniferous, and a vast thick- 

 ness of Subcarboniferous strata, most probably of Silurian age. The 

 Silurian beds usuallj' repose uncouibrmably on metamorphic strata, com- 



