GEOLOGICAL SURVEY OF THE TEERITOEIES. 41 



SO that the eroded materials of one locality were swept far away to 

 widely separated localities. Therefore, the superficial deposits of the 

 mining districts, which are usually very extensive, have their origin 

 in the immediate districts where they are now found. We may take 

 as an illustration the Alder Gulch, which is about twelve miles in 

 length, and varies from an eighth to half a mile in width, and is literally 

 filled up with sand, gravel, and bowlders, all of which were derived from 

 the mountains in the immediate vicinity — indeed, within the limits of 

 the drainage of that gulch. We may thus determine with a good degree 

 of certainty that, when we find placer-diggings, the source of the gold 

 thus found is not far distant, and is most probably within the limits of 

 the drainage of that locality. The origin of the i^lacer-gold is undoubt- 

 edly due to the erosion of the rocks in which it was originally precipi- 

 tated ; and inasmuch as the gold, so far as we now know, is found 

 altogether in the gneissic strata, its existence in the various gulches, 

 among the sand and gravel, is due to the grinding up by water of the 

 surface of the metamorphic rocks in the vicinity. Instances have 

 occurred where very rich placer-diggings have been found in gulches, 

 but the rocks which appear to have given origin to the float-gold, yielded 

 no rich lodes. This may be accounted for on the ground that the uj)per 

 l)ortions of the lodes contained all the rich ore, and that in the process 

 of erosion this ore was all ground up, while the remainder that is left 

 may have been lean, or even contained no gold at all. The principal lodes 

 that have been worked in the vicinity of Virginia City are near the head 

 of Alder Gulch, and are as yet only moderately successful. Up to this 

 date Montana seems to have gained its high state of prosperity princi- 

 j)ally from the richness of its gulch deposits. It is estimated that 

 $30,000,000 of gold have been taken out of Alder Gulch since its discov- 

 ery in 1863. The lodes all have a general strike northeast and southwest. 

 Perhaps they would be termed north and south lodes. I Avas informed 

 that all the lodes in the Territory have that general trend. The gangue 

 material is very similar to that in the gold lodes about Central City, 

 Colorado — quartz and feldspar of various textures. Sometimes the 

 gangue is very hard and compact ; again it is rotten quartz, as it is 

 termed by the miners. The country rock is mostly gneiss, also exhibit- 

 ing various degrees of hardness as to texture. The dip of the lode 

 matter is nearly west 50^ to 60°. The trend of the metamorphic strata 

 is about northwest and southeast. The Alder Gulch closes up in a ridge 

 of limestone, which forms a most remarkable wall, effectually shutting 

 off all communication with the Madison Valley to the east of it. The 

 altitude of Virginia City is 5,713 feet, while the head of the gulch is 

 about 500 feet higher, and around it a wall of limestone rises up 

 with its outcropping edges toward the gulch 800 to 1,000 feet, so that 

 this ridge is at least from 7,000 to 7,500 feet above the sea. From its 

 summit we can see at a glance, a broad extent of country. The Madison 

 Valley, with all its beauty of outline, is visible for thirty or forty miles, 

 while to the west and northwest the eye passes down the different 

 gulches and branches of the Jefferson Fork into that broad valley, over 

 the side ranges which intervene. We know that these limestones are 

 of Carboniferous age, and are a portion of the series that has extended 

 persistently all along our route from Salt Lake Valley, and perhaps 

 even the same great ocean bottom that extended, during that age, 

 over the area from the Mississippi Valley to the Pacific Ocean, and we 

 know not how much farther. As a general rule, these limestones always 

 contain a few fossils, enough to guide us in our wandering examinations, 

 but the rocks are usually so compact, and sometimes so much changed, 



