168 GEOLOGY OF THE YELLOWSTONE NATIONAL PARK. 



certain localities to the difficulties of determining structure. The later 

 intrusion of igneous rocks tended in some places to break up and confuse 

 the position of beds, but only in limited areas is such action especially 

 noticeable, and it may be said to have affected the larger mountain blocks 

 singularly little as regards the disturbance of beds. 



Sandstones of the Montana and Laramie formations constitute the more 

 elevated portions of the ridges. Both formations are conformable, and 

 throughout their entire development, from base to summit, the prevailing- 

 beds consist of a coarse yellowish-brown sandstone, of varying degrees of 

 purity. In the absence of a characteristic fauna discrimination between 

 the two formations is very difficult, and in most instances impossible with- 

 out much detailed work, with results not commensurate to the labor. 



Fort Pierre beds, which are mainly arenaceous, pass down into nearly 

 similar sandy deposits of the Colorado. The fauna which characterizes 

 these lower beds is one possessing a wide vertical range throughout the 

 Cretaceous sandstones, and the line of demarcation between the Montana 

 and Colorado is not always easy to draw. It is possible that beds provi- 

 sionally placed in the Montana may upon further investigation be found to 

 belong to the Colorado, the assignment being based upon their faunal 

 relations rather than upon their lithological habit. 



REGION OF WILDCAT PEAK AND HUCKLEBERRY MOUNTAIN. 



Snake River sharply defines this group of mountains on the west and 

 north. Above the broad valley of the Snake the mountains rise abruptly 

 over 2,500 feet in long rugged ridges whose outlines are more or less 

 obscured by dense forests. Northward, along the gorge of Snake River, 

 the mountain slopes are pi-ecipitous and in places rise like canyon walls 

 above the stream. 



Coulter Creek, named in honor of the distinguished botanist, Prof. 

 John M. Coulter, may be taken as the eastern boundary of these moun- 

 tains. It drains, by numerous tributaries, nearly the entire eastern slope, and 

 pours a large volume of water into the main stream, being the first impor- 

 tant affluent above the gorge of the river. The summit of the mountains at 

 their northern end, with an average elevation of 8,700 feet, presents a broken, 

 gently accidented country, easy to traverse and singularly attractive from 

 its park-like character. All the dominant peaks attain nearly the same 



