142 GEOLOGICAL SURVEY OF THE TEREITOEIES. 



tli^t it is not uncommon to find the Pliocene deposits inclining 5° 

 to 10°. 



Subsequent to these depositions, there was a period of intense volcanic 

 activity, in which the basalts were poured out over vast areas. We may 

 take, for example, the valley of the Jefferson, from the entrance of the 

 North Boulder into the Jefferson Eiver to Beaver Head Canon. On the 

 east side of the Jefferson a range of mountains extends along the valley 

 for thirty miles or more, with the northern portions of the west side 

 covered with a large thickness of Carboniferous limestones, like a steep, 

 flexible roof, the highest conical peaks rising to a height of 2,000 to 

 2,500 feet above the valley. At intervals of one to three or four miles, 

 these mountains are cleft from summit to base by small streams, forming 

 a gorge or canon of wonderful grandeur and picturesque beauty. The 

 stratified rocks thus reveal a dip varying from 45° to 60°, and apparently 

 pass down, curving under the valley and rising with a reversed dip on 

 the opposite side. The nucleus of all these ranges is, of course, a group 

 of stratified rocks composed of arenaceous clays, slates, quartzites, 

 micaceous gneiss, granulites, &c. A great variety of what I have termed 

 gneissic granites, granitoid rocks, granulites, metamorphic strata, &c., 

 occur. As a general rule, the Carboniferous strata repose unconformably 

 on this group of metamorphic strata ; but here and there, a perplexing 

 series of beds come in, quite varied in texture and occupying a restricted 

 area, but revealing no definite evidence of their age. That all the strati- 

 fied rocks known to exist in the Northwest, to the Lower Tertiary inclu- 

 sive, once extended all over this region, we have every reason to believe ; 

 but about the sources of the Missouri, Yellowstone, and Snake Elvers, 

 the Tertiary, Cretaceous, and Jurassic beds have been swept away, except 

 remnants exposed here and there. The Carboniferous groups, although 

 covering quite large areas, are not unfrequently seen capping the highest 

 mountains that suffered erosion to a tremendous extent. The occurrence 

 of rocks of Triassic age in the northwest is so problematical as yet, that 

 I do not now recognize them. Further investigations may bring to light 

 some evidence that will fix the position of the brick-red beds more posi- 

 tively, and until that time I prefer to include them with the Jurassic. 



The metamorphic group contains the" valuable mines of Montana. Not 

 unfrequently the most productive gulches are found, where the streams 

 have carved out a gorge through a thick series of Carboniferous lime- 

 stones, cutting deep into the metamorphic group. The volcanic action 

 seems to have taken place during all the later periods, continuing up to 

 the present time, and operating with greater or less force at different local- 

 ities. The above may be regarded as a brief summary of the princij)al 

 points in the geological structure of Montana and Idaho Territories. It 

 remains now to present an account of the local geology from point to 

 point, which must be a repetition substantially of this summary. 



The Pliocene deposits extend high up the valleys of the Pipestone 

 and White-Tail Deer Creeks, which are quite wide, with mountains on 

 either side. On the west side of the Jefferson, the foot-hills show a 

 great thickness, 600 to 800 feet. The silicified wood that is found 

 occasionally in these deposits is more beautiful than any I have ever 

 seen from any other formation. It is pure silica, and must have been 

 aided in its silicification by proximity to hot springs. Portions of it look 

 like opal or fine chalcedony, and in some portions the rings of growth are 

 well shown. As cabinet specimens they are especially sought for, and 

 must always be rare. The only other fossils known, are fresh- water and 

 land shells, and a few vertebrate remains. Organic remains of any 



