202 GEOLOGICAL SURVEY OF THE TERmTORIES. 



A natural division of the subject about wliicb information lias been 

 sought would seem to be; I, the minerals, and II, the mines of Colorado 

 and New Mexico; and these a^ain into 1. 1, the minerals of commercial 

 value, and I. 2, those of no commercial value, but more or less character- 

 istic of the rocks or formations in which they occur. 



The mining i}ortion of this report would have been better divided into 

 II. 1, gulch or placer mining, and II. 2, lode or legitimate mining, while 

 under the latter head the subject would naturally divide itself into a, the 

 methods in use for getting out the ore and taking care of the mines; h, 

 the dressing of the ores by mechanical processes ; and c, the chemical 

 treatcient of the ores, their reduction and preparation for the market, or 

 shipment out of the Territory. This would be a natural division of the 

 subject, but the time, and, consequently, the opportunities of observation 

 have been so insufficient for the above thorough treatment of the subject 

 that I have deemed it better to forward to you, as my part of the pre- 

 liminary report, only the notes 1 have made in the field, with a few 

 observations on various points connected with the subject. 



In a belt, of which it would be difficult to define the limits, but which 

 may be generally stated as lying east and west of the great continental 

 divide as far as the gneiss or granite extends, and reaching north an^ 

 south its far as investigation has made the Eocky Mountain chain known 

 to us, lie the ores of the precious, and some of the baser, metals. Of the 

 distribution of this great mineral wealth throughout the hundreds of 

 leagues of this belt very little is known, the small area which has become 

 the prize of the gold-seeker furnishing wholly insufficient data upon 

 which to base general conclusions. 



To begin with, the rock in which occur all these lodes is that which 

 carries the precious metals, with rare exceptions, the world over, and 

 which is either a granite or a gneiss, or, as in the Central City district, 

 such an inextricably confused mixture of both that it were impossible to 

 call it either. This is the country rock. Whether from the great changes 

 to which this rock has been exposed through countless ages, or whether 

 from other causes, it shows itself in most various forms at different 

 places, and passes by imperceptible phases through gneiss, granite, sye- 

 nite, and porphyry. This porphyry is perhaps more frequently observed 

 in the neighborhood of veins. 



A fine illustration of the irregTilarity with which these rocks succeed 

 each other is to be observed along the road from Mount Vernon through 

 Idaho City to Georgetown. Along Clear Creek, from Fall E.iver to 

 Georgetown, the inclination and direction of the rocks appear to be as 

 variable as their structural character, a general northwesterly dip being 

 perhaps most common, while red and gray, heavy-bedded, and thinly- 

 laminated gneiss and red and gray granite succeed each other in utter 

 confusion. Here and there a vein of quartz or quartz-porphyry or sye- 

 nite (very frequently auriferous) is visible, forming a light-colored streak 

 usually. do v>'n the sides of the opposite hills. This composite character 

 of the country rock has been noticed, as I am informed, in most, if not 

 all, of the mining districts, and on both sides of the Sierra Madre or main 

 range. The gangue rock is most frequentty quartz, which, of course, 

 assumes very different appearances at different places, both in texture 

 and in color. In some cases the gangue rock is porphyry more or less 

 weathered. (Brown Lode, West Argentine, ct al.) 



The minerals of Colorado of commercial value which are most widely 

 distributed are auriferous iron and copper i)yrites, (malachite and the 

 sulphates of iron and copper from their decomposition, though nowhere 

 in large quantities, being spread over wide areas,) zincblende, argent- 



