62 GEOLOGICAL SUEVEY OF THE TEEEITOEIES. 



in texture from a coarse sandstone to a compact homogeneous quartzite. 

 There is in all the rocks a small per cent, of lime. The height of 

 the hluff-like wall is about 500 feet, and on the summit there is an ir- 

 regular bed of basalt, which fractures into an imperfect columnar form. 

 In other localities layers of basalt are intercalated with the sedimentary 

 beds, effecting greater or less changes in the contiguous rocks. Again, 

 the basalt has flowed to the surface through the underlying strata, and 

 spread over restricted areas. This group of rocks is remarkably well 

 developed, and occupies nearly all the interval between the belt or ridge 

 of limestone extending from near the junction of the Three Forks south- 

 westward to the Yellowstone Eiver and Shield's Eiver. From the 

 agency, this group extends down the Yellowstone as far as the eye can 

 reach, so that there is a belt here of at least fifty miles from north to south, 

 and twenty from east to west, which may be said to be almost entirely 

 occupied by these beds, mingled with basaltic rocks which have been 

 effused at different periods, and have been cooled under varying con- 

 ditions. The same group of rocks appears on the right side of Gar- 

 diner's Eiver, forming a bluff wall 800 to 1,200 feet high, with the same 

 irregular beds of basalt. Similar steel-gray rocks occur in the Middle 

 Park, containing leaves of deciduous trees, with thick beds of basalt, 

 inclining at a high angle, in conformity with the Tertiary and Cretaceous 

 beds, I have called these steel-gray beds Cretaceous and Tertiary, and 

 yet I do not positively know that any portion belongs to the Tertiary. 

 It is the group of rocks that contains the coal in this portion of the 

 west. There are coal-beds near Port Ellis, and indications of coal near 

 the mouth of Shield's Eiver on the Yellowstone. Leaves of deciduous 

 trees of Tertiary affinities are abundant. ]S'o molluscan fossils were 

 found, yet the character of the rocks and their great thickness leads 

 me to believe that they are Upper Cretaceous, passing up without any 

 l>hysical line of separation into the Lower Tertiary". I think, also, that 

 they form a part of the same group which contains the coal on the 

 Lower Yellowstone, below the mouth of the Big Horn. These forma- 

 tions about the sources of the Missouri Eiver and its branches need a 

 much more careful and extended study than I have been able to give 

 them, and I can only look forward into the future with hope, for time 

 and opportunity to group them in their proper position. 



The ridge of limestone which crosses the Yellowstone at the lower 

 caiion seems, to one looking from the valley below, to rise abruptly out 

 of the i)lains ; the ridges, which are made up of the Jurassic, Cretaceous, 

 and Tertiary groups, incline at various angles from the main ridge, and 

 seldom rise above the general level more than 100 or 200 feet, while, at 

 the base of the ridge, the u^jturned edges of the Lower Cretaceous and 

 Jurassic rocks extend in long lines across the Yellowstone as far as the 

 eye can reach, but not rising above the general level of the plain more 

 than 50 or 100 feet, and sometimes not at all, but so covered with debris 

 that they are only exposed in the channel of the Yellowstone. But the 

 beds of limestone and quartzite of the Carboniferous group rise up 800 

 to 1,200 feet above the valley below, and though the inclination in the 

 caiion is only about 15° to 30", yet the outer beds dip 60° to 80°; this 

 difference is not due to any want of conformability in the series, but 

 doubtless to the greater ease with which the more modern beds have 

 yielded to the erosive forces, while the Carboniferous limestones and 

 quartzites have most effectually resisted those agencies. On the Yellow- 

 stone the lower ridges extend far to the northeast, with a somewhat 

 irregular height, while the limestones are elevated so as to form a group 

 of lofty peaks nearly as high as the volcanic cones of the snowy range, 



