2 GEOLOGY OF THE YELLOWSTONE NATIONAL PARK. 



Madison rivers, the watersheds between which meet one another in Three 

 River Peak. The special description of the physiographic features of the 

 region, however, including the glaciation, will be found in Part I of this 

 report, where it is treated by Mr. Hague. Without entering into a topo- 

 graphic description of the Gallatin Mountains, it will be in place here 

 to call attention to the fact that 4he region in question, within the Park 

 boundary, is a block of country delimited on the east and on the west by 

 profound faults trending nearly north and south, the western fault line 

 having a somewhat northeasterly trend. This block, about 7 miles wide, 

 is bounded on the south by a capping of lavas, which borders it also to 

 some extent on the east and on the west. The northern end of the block 

 lies beyond the Park boundary, in the neighborhood of Cinuabar Mountain. 

 The block is wider at the south, and narrows northward. It is a wedge- 

 shaped mass cut diagonally across a synclinal trough, with one long and 

 one very short limb. The latter appears for only a short distance at the 

 northern end, in Cinnabar Mountain. Within the area of the Park the 

 block has the structure of a monocline, dipping northeast across the longer 

 diameter of the block. Minor faults and folds modify the structure some- 

 what and introduce local complications, which will be described in detail. 



As a result of the dipping of the block to the northeast, the oldest 

 formations are found at the southern and southwestern ends, and the 

 youngest formations at the northern. The rocks are well exposed, the 

 succession of the strata is clearly made out, and the form and character of 

 the igneous material that has been forced through the sedimentary rocks 

 are readily observed. The study of the igneous bodies and their relations 

 to the geological structure of the block proves that the dynamic history of 

 this particular area was complex, and extended over a long period after the 

 deposition of the coal-bearing Laramie sandstones. In fact, a succession of 

 dislocations must have followed one another through the greater part of the 

 Tertiary period. This will appear from the description which follows. 



Erosion has carved deeply the surface of this upturned, fractured, and 

 distorted block, grooving it with A-alleys and gulches, the eastern system 

 trending northeast and east and draining with the dip of the strata, the 

 western system trending and draining northwest along the general line of 

 the strike and being in all probability controlled by lines of fracture in 

 this direction. The intervening elevations rise abruptly to sharp peaks 



