1871.J (389) 



PROGRESS IN SCIENCE 



MINING. 



The recent development of the extraordinary mineral districts in Nevada 

 and Colorado, now rendered accessible by the Pacific Railroad, has led to an 

 official exploration of the chief ore-bearing regions in these States. In 1867 

 Mr. Clarence King was appointed Geologist in charge of the expedition, and 

 the volume of the Report which relates to Mining, prepared chiefly by Mr. J. D. 

 Hague, has recently been published.* The complete report will comprise five 

 volumes — two on Geology, one on Mining, and the others on Zoology and 

 Botany. The third volume — that on Mining — has been issued first in conse- 

 quence of its great practical importance. It forms a magnificent quarto volume 

 amply illustrated, and accompanied by a large folio atlas of maps, plans, and 

 sections, printed in colours. 



Although professedly an exploration of the fortieth parallel, the survey has 

 been extended to a belt of country stretching for about 100 miles from north 

 and south, and included between the meridians of 109° and 120 ; moreover, 

 any important mining districts lying adjacent are also noticed. 



Over the whole Cordilleras deposits of metallic ores are more or less abun- 

 dant; these deposits usually range in longitudinal zones parallel to the general 

 trend of the rocks, which is nearly north and south. The Pacific Coast Range 

 upon the west carries ores of mercury, tin, and chrome-iron ore ; the next belt 

 — that of the Sierra Nevada and Oregon Cascades — bears on its western slope 

 copper and gold, while along the eastern base is a chain of silver mines; these 

 silver deposits extend into Mexico, whilst the gold ores — both auriferous veins 

 and placer workings — run far into Alasca. Through Middle Mexico, Arizona, 

 Middle Nevada, and Central Idaho is another range of silver-mines; and 

 through New Mexico, Utah, and West Montana a zone of lodes containing 

 argentiferous galena ; whilst on the east a belt of gold deposits runs through 

 New Mexico, Colorado, Wyoming, and Montana. For a proper understanding 

 of these mineral deposits, it is of course needful to study the geology of the 

 far West. A series of conformably stratified beds, varying in age from early 

 Palaeozoic to late Jurassic times, appear to have been at the latter period 

 upheaved and corrugated, whilst masses of granitic and porphyritic rocks were 

 injected into the fissures formed by this disturbance. Deposits of cretaceous 

 and tertiary age were then laid down on the flanks of these rocks, and at a 

 period subsequent to the miocene epoch they were folded into mountain- 

 chains parallel to, but outside of, the earlier system. This second upheaval 

 was accompanied by volcanic disturbances, the lavas overflowing and covering 

 the earlier deposits. The mineral lodes of these regions consequently resolve 

 themselves into two distinct sets, corresponding to the two periods of disturb- 

 ance — firstly, those of Jurassic age, consisting of veins of granite, or in more 

 or less metamorphic strata, varying in date from Jurassic to Devonian ; and, 

 secondly, veins of tertiary age in volcanic rocks. A large number of lodes are 

 referable to the first date, but many of the richest, such as the Comstock 

 lode, are referable to the second disturbance. 



As might be expected, a large proportion of the volume is occupied by a 

 description of the celebrated Comstock vein. This lode courses nearly 

 N. and S., and has been traced for a distance of four miles, the part already 

 developed occupying about two miles. The masses of ore occur in discon- 

 nected groups, and the width of the vein varies considerably, sometimes con- 

 taining masses of vein-stuff 100 ft. in width. The report contains a minute 

 description of the lode, with details of the many mines opened upon it, and of 



* Report of the Geological Exploration of the Fortieth Parallel ;. vol. iii., Mining In- 

 dustry, by J. D. Hague, with Geological Contributions by Clarence King. Washington, 1870. 



