GEOLOGICAL SURVEY OF THE TERRITORIES. 



47 



iferous limestones underlie the greater portion of the country east of 

 the Grand Caiion and the lake, extending over to the sources of Clark's 

 Fork, Stinkingwater, and other streams. This enormous thickness of 

 conglomerate once extended all over it in one unbroken mass, but since 

 the subsidence of the waters the volcanic mass has been cleft down 

 into the underlying beds of limestone in every direction, forming some 

 as remarkable scenery as there exists on our continent. Mr. Uolmes, 

 the artist of the expedition, has sketched many natural sections show- 

 ing most clearly the relations of the conglomerates with the limestones 

 below, and presenting in detail the singularly unique forms which ai;p 

 produced abundantly by the meteoric forces. Mingled with the conglom- 

 erates are vast irregular masses of trachyte, the result of repeated over- 

 flows from the craters and fissures. In many cases the nucleus of the 

 cones or peaks are massive trachyte. There are also alternate beds of 

 trachyte and conglomerate. 



About the sources of the Middle Fork and Clark's Fork are some very 

 interesting silver-mines. They are all located in the limestones, from 

 6,000 to 7,000 feet above sea-level. These limestones rest uj)on granites 

 near Clark's Pass, and one poor mine has been found in those rocks. 

 Tbe " croppings " are of a rusty yellow color, a sort of rusty limestone 

 or qnartzite, which can be traced for miles. The mines have not yet 

 been developed to any extent, but the ore, which is galena, looks well. 

 I think the ore is located in regular fissure-veins, extending downward 

 about at right angles with the strata, (which are nearly horizontal,) and 

 probably reach down into the raetamorphic rocks beneath. There is 

 the greatest abundance of water and wood in this region to work these 

 ores should they prove of any value ; but the mines are at present so 

 diflhCult of access, so far from market, and the seasons are so short, 

 that their value is nominal at this time. Still, the rapidity with which 

 this western country develops under the stimulus of rich mines and 

 railroads is so great that these far-away ores may become valuable 



Fig. 8. 



f -f-w 





INDEX AND PILOT PEAKS. 



sooner than we could anticipate. A regular mining-district hns been 



formed here and numerous lodes staked out. (See map of East Fork.) 



Passing the divide, we ascended the mountains which give origin to 



