82 GEOLOGICAL SURVEY OF THE TEREITORIES, 



still sustained more or less by them. The limestone-ridges, which can 

 be seen on either side of this broad valley, are only remnants of what 

 must once have extended over a large area of country. In the ridge 

 west of the town of Eadersburgh the lower strata are inverted, inclining 

 past a vertical 45°, while the Carboniferous limestones stand nearly 

 vertical, though the quartzites and red sandstones of the Jurassic are 

 either vertical or incline past 5°. These red Jurassic beds I think are 

 the same as those shown along the banks of the Missouri, at the Great 

 Falls. The thickness of these beds is greatly increased as we proceed 

 northward, and at this point must be 1,000 to 1,500 feet thick. The 

 sedimentary beds extend to Indian Creek, and then suddenly disappear, 

 and then along the flanks of the mountains on the west side of tlie road 

 only a great thickness of drift-material is seen. The little streams, 

 which have worn deep gulches into the mountain-sides, have also worn 

 channels through the drift, exposing its thickness and character very 

 clearly. 



The placer-mines are very extensive. Some valuable silver-mines 

 have been discovered in the mountains. The Missouri Valley here 

 is about twenty to twenty-live miles in width, with high ranges of 

 mountains on both sides. From Eadersburgh the road passes over the 

 vertical edges of the dull, purplish Jurassic beds for twelve mil6s, to 

 Indian Creek. The drift is made up of rounded bowlders mostly, and 

 must be 300 to 500 feet thick. Extending eastward toward, the Mis- 

 souri, in the belt of sedimeutary beds between Eadersburgh and Indian 

 Creek, may be seen a rather level, rounded, cretaceous hill, so grassed 

 over that few out-croppings could be observed. I had very little 

 opportunity of examining the rocks about Helena, but believe that they 

 are mostly granitic, capped here and there with strata of the Silurian 

 age. It is also probable that there are remnants of Carboniferous beds 

 in some places in the vicinity. 



In j)assing along the stage-road a little west of south from Helena to 

 the valley of the Jefferson Fork, a few patches of the older Silurian 

 beds are seen, while among the low hills the lake-deposits show that the 

 entire country was a vast fresh-water lake at a comparatively modern 

 period. The high hills on either side of the road are weathered into 

 curiously rounded forms and covered with grass. Prickly Pear Caiion 

 is a remarkable district for placer-mines. Water seems to be abundant. 

 The rocks are mostly rusty-brown gneisses, weathering into forms much 

 like those in the Laramie Eange near Sherman, Union Pacific Eailroad. 

 These granitic rocks extend to the source of Prickly Pear Creek, 

 and the liigh hills on either side are covered thickly with pines. Jef- 

 ferson City is located among the reddish-granitic hills. From the head 

 of Prickly Pear Creek we crossed the divide to the valley of North 

 Bowlder, and in the valley on either side the massive granites rise in 

 low, singularly-shaped columns, piles, &c., giving to the region the ap- 

 pearance of old ruins. Wherever gold-mines are found, whether in the 

 lodes or gulches, we may be assured that the gneissic ropks are exposed. 

 We have up to this time been able to do little more than make a recon- 

 naissance of one of the most interesting and instructive portions of the 

 West, in a geological point of view. The reports of the surveys for 1871 

 and 1872 can certainly claim to be valuable contributions toward the 

 geology of Montana and Idaho, and at some future period, when the coun- 

 try has become more easily accessible, the work may be resumed and 

 carried on to completion. 



We have thus far attempted to describe briefly the main geological 

 features of the district explored by the party during the past season. 



