GEOLOGICAL SURVEY OF THE TERRITORIES. 83 



Nowhere in Montana have I found the anticlinal folds or the synclinal 

 valleys so distinct!}' defined as in the interior basin of Utah. StilJ, 

 there are frequent local synclinals and anticlinals, as we find them 

 developed below tlie Three Forks. The prominent features, however, are 

 the widely-extended areas of elevation, though a single anticlinal may 

 embrace several a])parently distinct langes of mountains. The operations 

 of the survey during- the past season more strongly convince me of the 

 position that I have so often taken in my reports, of the originally wide- 

 extended and continuous character of the entire group of sedimentary 

 strata ; that where that continuity is broken it is the result of upheaval 

 attended with erosion. It is possible that the later Tertiary group may 

 not liave been continuous, but existed in basins. But from the Silurian 

 to the Upper Lignite group, inclusive, a thickness of 10,000 to 15,000 

 feet extended, in an unbroken, horizontal mass, over nearly or quite the 

 entire area of Montana, and probably much more widely ; and that 

 what we find remaining at the present time are only remnants of this 

 vast mass. Occasionally the entire series of formations is exposed, as 

 ia the East Gallatin Eauge, where for twenty miles, on a line from east 

 to west, the entire series of sedimentary strata may be seen from the 

 Silurian to the top of the Lignite group in consecutive order. (See Fig. 21.) 

 The groups of limestones and other rocks, as we see them inclining from 

 the Yellowstone Eange, in the Yellowstone Valley, show that they once ex- 

 tended uninterruptedly over the entire area, where now mountain-peaks 

 rise amid perpetual snows, 11,000 feet above the sea. The Silurian group 

 increases in importance as we proceed northward from the Three Forks, 

 and southward from that point it diminishes in thickness and changes 

 very much its mineral texture. Toward the south we find little of 

 the thin shaly and mud layers with the variegated sandstone, but in 

 their stead, a quartzite passing up into a very hard, brittle limestone. 

 Still, we believe that this group in some form and with greater or less 

 thickness underlies the greater part of the Eocky Mountain region. 

 About the Black Hills of Dakota and the Big Horn Eange, the Potsdam 

 group presents a different mineral structure from the rocks of the same 

 age about the sources of the Missouri. 



The Carboniferous group, like most sea-deposited rocks, is very widely 

 distributed. It is probable that it will eventually appear that this 

 group of beds, as it is known, west of the Mississippi v/ill be found to 

 cover a wider extent of territory and to maintain a more uniiormly 

 similar mineral texture than any other formation in the scale. By 

 reference to the list of fossils prepared for this report by Mr. Meek, it 

 will be seen that, even in the most widely-separated localities, there 

 is a similarity or identity in the organic remains. OKI Baldy, at the 

 head of Alder Gulch, forms a portion of a limestone-ridge in which the 

 series of beds is shown with a vertical thickness of 1,000 to 2,000 

 feet and extends .off to the soutlnvest, giving origin to the Stinking 

 Water, Black Tail Deer Creek, Eed Eoek Creek, and mauj' others. 

 These beds have a general dip to the southeast. We believe also that 

 the Juras'Sic and Cretaceous group had a very wide extension, tliough 

 l>erhaps not as great as tliatof the older loruiations. They have been 

 more extensively worn aw.iy so that at the i)resent time they occur in 

 fragments among the uphe;ived mountain-ridges and covering restricted 

 areas. So far as the position of the sedimentary rocks is concerned, 

 they may occur at any elevation. The beds may pass under the lowest 

 valleys or be found capi)ing the gneissic roclTs upon the summit of the 

 highest mountains. Tliis is certainly not due to any inequality of the 

 surface of the gneissic rock prior to tlie deposition of the succeeding 

 beds, but unquestionably to upheaval. 



