356 EEPOET UNITED STATES GEOLOGICAL SURVEY. 



Tlie above section lias a linear extent of about four miles between the 

 forks of Gray's Creek. Altliougli tlie deposits in the immediate south- 

 west slope of Station XVII are somewhat obscure, and withal so altered 

 by metamorphic action as render their examination difficult, yet their 

 more favorable exposure in the opposite hill-side to the west affords satis- 

 fa«i»(:ory data for the determination of the relations of the sedimentaries to 

 the volcanic phenomena with which they are here associated. The igne- 

 ous mass protruding in the crest of the ridge seems to have been forced 

 up nearly in a vertical direction, carrying the sedimentary beds up with 

 it instead of fracturing them at once, so that at the extremities of the 

 upthrust they were not rent apart. But at the point of greatest tension 

 they were j)artially fractured, the igneous matter following the crevice 

 thus produced, as a wedge-shaped mass, which subsequent erosion has 

 bared, and thus revealed the origin of the little anticlinal fold, of which it 

 forms as well the nucleus. The apparent identity of this eruptive rock 

 with that prevalent in the Elk Mountains of Western Colorado, should 

 also be mentioned, specimens from these remote regions showing slight 

 if any difference in their mineral constitution. 



The sedimentaries in Station XVII ridge, as before mentioned, may be 

 traced to the northwest, where they reappear in the vicinity of Station 

 XVI, in which latter quarter, however, the series belongs to the south- 

 west side of the present uplift. From the latter station, looking across 

 the broad valley of Willow Creek, beneath the greatly uplifted volcan- 

 oes at the northern end of the Blackfoot Eange, a considerable belt of 

 reddish-tinged strata appears in the northeast flank of that range, out- 

 lying the Carboniferous in Blackfoot Peak. These deposits, as seen at 

 a distance, present i^recisely the appearance of the strata exx)osed in the 

 Willow Creek ridge above described ; and continuing the north of west 

 strike of the latter beds across the intervening plain where they are con- 

 cealed from view by the basaltic flow, it would intersect the Blackfoot 

 Eange at the point where these deposits have their apparent counterpart 

 in the foot-hills east of Blackfoot Peak. The trend of the eruptive mass 

 in Station XVII is exactly in hue with Mount Bainbridge (Station 

 XXVIII), IG miles to the southeastward, where mineralogically identical 

 igneous eruptions are again met with, which will be noticed in a subse- 

 quent section of this chapter relating to the Caribou Eange. The 

 banded reddish and drab appearance of the outcrop of the sedimentary 

 deposits of the present group of hdls is repeated also in the foot-hills 

 northwest of Mount Caribou and it seems very probable that the pres- 

 ent uplift has intimate connection with the remarkable phenomena ob- 

 served at that locality. These deposits may be described as variegated 

 red and drab beds, the appearance they usually present in distant- viewed 

 exposures. They form the low swells into vt^hich the southeast end of Sta- 

 tion XVII ridge dies away, as also the similar terminus of the main 

 Willow Creek ridge, as previously mentioned. In the undulating border 

 hills to the northeast of the confluence of the forks of John Gray's Creek 

 the same series of strata is also seen, where they are dominated by various 

 levels of the upraised volcanics which rise up in abrui)tly terminated 

 benches on the flank of the Caribou Eange in that quarter. But every- 

 where in the lower depressions separating these low basin ridges, the 

 sedimentary deposits of which they are largely comi^osed are concealed 

 beneath the basaltic flows, though their continuity scarcely admits of a 

 doubt, however isolated their present manifestation. 



Beyond the low basaltic plateau which envelops the southern end of 

 the Willow Creek ridge, the surface again rises into a low ridge extend- 

 ing southeastward several miles into the adjacent district, and which 



