332 GEOLOGICAL EXCURSION TO THE fiOCKT MOUNTAINS. 



These rocks continue to form high mountains along the northern side 

 of the Yellowstone River east of Gardiner. The peaks surrounding the 



head of the valley of Bear < 'reek opposite Gardiner are volcanic ejecta 



mcnta and intruded bodies of porphyrite. The bench at the base of 

 Sheep Mountain, about 000 or 800 feet above the river, consists of a lava 



How of basalt resting on the upturned edge of faulted Cretaceous sand- 

 stones. Small wedges Of these jocks have Settled Short distances and 

 lowered parts of the basalt sheet to different levels, so that it appears 

 from tin; south side of the river as though there were a number of 

 superimposed sheets. The light-colored deposil upon the basalt is the 

 remains of an ancient travertine, similar to that at Mammoth Hot 

 Springs. A profound fault passes along the south base of Sheep Moun- 

 tain in a southeast and northwest direction, the displacement being 

 over 0,000 feet. This fault is shown in the cross section through Emi- 

 grant Peak and the summit of Sheep Mountain ( Pig. 14-c). The geologi- 

 cal structure is exceedingly simple and is largely a repetition of that in 

 the second section (Fig. 14-u). The northern cud of the section crosses 

 the Yellowstone River two miles above Chickory Station, where it prob- 

 ably intersects the Mill ('reek fault. 



[By Walter h. Weed.] 



At Cinnabar Mountain the sedimentary rocks are again met with, 

 a section being exposed from the Archean to the Laramie Cretaceous. 

 The mountain received its name from a prominent band of bright red 

 sandstone, the so-called Devils Slide, there being of course qo mercury 

 there. In the mountain the sedimentary Strata are nearly vertical, 

 being- the sharply Upturned end of a synclinal trough whose axis is the 

 sag south of the mountain. It is the most convenient locality for the 

 traveler to examine the stratigraphical section, as the rocks are well 

 exposed and readily accessible. The northern pari of the mountain is 

 composed of Paleozoic limestones so closely compacted that the subdivi- 

 sions are not easily recognized, but the quartzite at the summit of the 

 Carboniferous, witli its red magnesian limestones, is distinctly differen- 

 tiated. Above these beds are the Triassic sandstones forming the 

 Devils Slide, and the ripple-marked quartzite which overlies them 

 forms the north wall of the most prominent of the gulches that seam 

 the mountain side. The gray Jurassic shales are well exposed and 

 contain an abundance of fossils characteristic of the Rocky Mountain 

 Jura, such as Myaoites, Ixhynchonella, Gryphasa, Camptoneetes, etc. 

 It is in these rocks that the intrusives forming the south wall of the 

 great gulch have been injected. Overlying these Jurassic beds are the 

 grits and conglomerates of the Dakota Cretaceous, in which a limestone 

 belt carrying fresh-water fossils may be seen. Above tins the dark 

 bituminous shales of the Fort Benton, with occasional arenaceous belts, 



