CHAPTER IV. 



DESCRIPTIVE GEOLOGY— SKAKE RIVER DRAINAGE AEEA. 



John Dat's Rivee — Salt River Range — Salt River — Blackfoot Basin — Port- 

 neuf Eivee — Marsh Creek — Portneuf Range — Bannack Range. 



In the present chapter, I shall take up the general geology of the 

 branches of Snake Eiver that drain the northwestern portion of the dis- 

 trict. As there are alternately streams and mountains, in general par- 

 allel to each other, I shall take them up in order from east to west instead 

 of considering the drainage system first and the mountain ridges and 

 ranges afterward, as was done in the preceding chapter. I shall endeavor, 

 ih taking up each stream and range, to give a general idea of the geo- 

 logical structure of the immediate region as determined by the facts noted 

 by us. These, however, must be necessarily incomplete. The whole 

 region is one admirably adapted to rapid topographical work, while the 

 geology is complicated, especially in the ranges. The general structure 

 was well determined, but there are many detailed points still to be worked 

 out, and a geologist will find much to interest him for a long time, espe- 

 cially in the Salt Eiver Range and in the Preuss Range. 



The branches of Snake River draining this portion of the district are 

 John Day's River, Salt River, Blackfoot River, and the Portneuf River. 

 Salt River Range is the highest mountain range. Other ranges are the 

 Portneuf, Bannack, and. the northern spurs of the Preuss Range. 



JOHN DAY'S RIVER. 



John Day's River has a length of about 35 miles within our district. 

 Its course is about due north to within a couple of miles of our north 

 line, where it turns to the northwest. The river was named after one of 

 the early trappers of this region. Its extreme head is in the southern 

 end of the Wyoming Range, between La Barge Creek and the branches 

 of Piney Creek. Its valley is narrow and canon-like throughout its 

 length, and the hills sloping to it are well timbered with pines. The 

 stream is clear and rapid, generally difficult to ford. Geologically the 

 valley is a monoclinal cut in rocks of Jura-Trias and Cretaceous age. 

 Opposite Station 55 the river flows over the red quartzitic sandstones 

 that he just above the limestones of the Wyoming Range. Tfiey out- 

 crop on both sides of the valley, dipping 35° or 40° to the westward. 

 The Wyoming Range on the east side of the river presents an almost 

 vertical face toward the valley. The summit is capped by " red beds " 

 in almost horizontal position. Below the red sandstones are conform- 

 able limestones, which, as we get nearer the level of the valley, curve and 

 dip conformably beneath the red sandstones that form the hogback-like 

 ridges on its east side. The projecting angle of the fold has therefore 

 been eroded away. In the valley the river winds but little. A couple 

 of miles above Station b the river turns slightly to the eastward out of 

 the red sandstones, but soon turns westward again, cutting across the 

 red sandstones into the gray Jurassic beds that lie above. The stream 



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