276 GEOLOGICAL SURVEY OF THE . TEREITOKIES. 



CHAPTEE I. 



PEELIMINAEY EEPOET UPON THE MINING-DISTEICTS OP COLORADO, 



According to instructious received, I visited the mining regions of 

 Gilpin County, Colorado, abont the middle of May, 1873, and subse- 

 quently those of Clear Creek and Boulder Counties. The object in view 

 was a thorough recognition and definition of the geological and miner- 

 alogical relations shown by the lodes of these localities to each other 

 as well as to the surrounding rock. Owing to the early season, a num- 

 ber of mines that afterward were worked, were not then in operation j 

 and as a personal visit to all, or even only the greater portion of the 

 mines, was entirely out of the question, such were selected for examina- 

 tion as were acknowledged to be the most important ones of the district, 

 besides those possessing any particular interest from a geological or 

 mineralogical point of view, the acquaintance witli which seemed neces- 

 sary for a successful completion of the woijlj undertaken. It is mainly 

 of the mines visited personally that I shall speak in the subjoined 

 pages. 



The la-ck of statistics, which may be considered very essential in a 

 mining-report, is to be accounted for by the fact that this report pre- 

 tends only to examine the geological and mineralogical relations of the 

 ore-bearing veins of those districts. As a large percentage of the min- 

 ing population follow this occupation upon claims in their own posses- 

 sion, for the purpose of> making a livelihood, the facilities for obtaining 

 statistics are by no means satisfactory, and it would require more time 

 than I could spend to compile thorough statements of production, re- 

 turns, &c. A carefully-compiled statement of the work done at the 

 Monte Cristo mine has been very kindly allowed me for publication by 

 Mr. Mills, of Central. From it can be seen an account of the actual 

 cost of working a mine on a small scale, and calculations for more ex- 

 tensive operations may be made accordingly. 



Neither historical notes nor descriptions of machinery employed have 

 been given in the report^ as both have been treated of in Mr. Hague's 

 excellent report upon the mining regions of that section. Merely the 

 gecgnostic, geological, and mineralogical features of the localities under 

 consideration, and their mutual relations, are dwelt upon. It is hoped 

 that by presenting this character of the mining regions, which has not 

 been sufficiently done before, more light may be thrown upon their merits 

 or demerits as such, and more definite knowledge gained regarding their 

 constitution. 



In order to facilitate the ready comprehension of terms used almost 

 exclusively by miners, and in order to give a more definite idea of the 

 questions involved in the subjoined report, a short introdnction is given, 

 treating briefly on the subject of ore-deposits. 



The substances mostly occurring in deposits that are sought after by 

 man are gold, silver, platinum, copper, lead, tin, zinc, iron, coal, salt, 

 bitumen, petroleum, and others, existing partly in their native state, 

 partly in combination with other substances. Two great divisions are dis- 

 tinguished, and imbedded deposits— superstratoid deposits ; the former 

 being partially or entirely surrounded by a valueless material, the 

 " country ; " the latter deposited upon the surface, and containing its 

 valuable minerals partly as float, partly as a superficial deposit. Im- 



