GEOLOGICAL SUEVEY OF THE TERRITOEIES. 



39 



WEATHERED GRANITES AT MADISON CANON. 



tliickness of black micaceous gneiss, with seams of wMte quartz, the 

 coarse feldspathic granites, literally an aggregate of large crystals 

 of quartz and feldspar, then 



underneath the black gneiss Fig. 7, 



again. In this canon there is 

 a most interesting illustration 

 of the weathering of the red- 

 dish feldspathic granites by the 

 peeling off in thin concentric 

 layers, or as I have denominated 

 it in my former reports, disin- 

 tegration by exfoliation. I 

 have never observed a more 

 marked example anywhere in 

 the West, and Fig. 7 shows it 

 well. After passing through 

 the canon a distance of about 

 three miles, the road bends 

 to the north, leaves the valley of Stinking Water, passes over a 

 high divide to Alder Gulch, in which Virginia City is located. On the 

 right or east side of the road, the rather rounded and, in some instances, 

 grass-covered hills, continue all the way. On the left or west side, the 

 gneiss and quartzite continue for a short distance, when the mountain 

 range, which has hitherto walled us in on the west side of the road, bends a 

 little to the northwest, and extends to the Jefferson Valley, parallel with 

 the Stinking Water, and rises quite abruptly, 2,000 feet above thechannel 

 of the stream. The base of this ridge or range is a smooth lawn-like slope, 

 down to the margin of the stream, while the ridge itself is composed of 

 massive beds of limestone inclining 60° to 70^*, the outcropping edges 

 projecting sharply on the summits, and the northeast sides sloping down 

 into the plain, like a very steep roof. The valley itself is a beautiful 

 and fertile one, and is one of the numerous valleys that open into the 

 Jefferson Fork. It will average from four to six miles in width and 

 about twenty miles in length below the canon, and is covered with a 

 moderate thickness of the Pliocene deposits. On the east side of Stink- 

 ing Water, the rocks are entirely composed of gneiss, of the usual va- 

 riety of texture and composition, the strata inclining southwest at 

 various angles, so that the Stinking Water really flows through a 

 synclinal valley from the canon to its junction with Jefferson Fork. In 

 the valley and among the foot-hills of the mountains, are here and there 

 patches or remnants of the great basaltic crust that must at one time 

 have extended over most of the area occupied by the valleys. From the 

 Stinking Water to Virginia City, a distance of about ten miles, the rocks 

 observed were of metamorphic origin, with here and there indications of 

 the effusion of basalt. 



Virgmia City is located in the center of one of the richest mining dis- 

 tricts of Montana, and a description of the surrounding country would 

 apply, in most particulars, to- all the mining portions of the Territory. 

 The precious metals, as gold and silver, are found, so far as my ob- 

 servations have extended, entirely in the metamorphic rocks which hold 

 a position below all groups of strata that we have been in the habit of 

 regarding as Paleozoic. Whether they belong to the series denominated 

 in Canada the Huronian or Laurentian, we have no data to decide posi- 

 tively ; but inasmuch as they are all clearly stratified rocks, they are plainly 

 of sedimentary origin. These rocks underlie the entire country west of the 

 MississipiDi. We may safely assume this position whether they are vis- 



