HEAT — VEINS. 339 



1. Tlie upper intersected rocks of difficult corrosion. — These rocks are of 

 any kinds not calcareous : as shales, sandstones, or other related fragmental 

 kinds, or, but much less frequently, crystalline rocks. 



The famous copper mines south of Lake Superior are an example. The 

 upper intersected rocks are sandstones, conglomerates, and tufas. The igne- 

 ous rock is mainly of the basaltic type. The copper is native copper con- 

 taining generally 3 per cent of silver, and occasionally speckled with silver. 

 It occupies irregular fissures and cavities in the igneous rock, especially its 

 amygdaloidal varieties, and also occurs in the adjoining sandstone. It some- 

 times constitutes amygdules, has often a gangue of zeolites, or coats 

 crystals of analcite and quartz-crystals, and thus it proves its contempora- 

 neous origin with these materials. One great sheet of copper was 40 feet 

 long, 6 feet wide, and 6 inches thick, and weighed, by estimate, 200 tons. The 

 conditions show that the copper came up along with abundant moisture from 

 some deep-seated source. In 1891, the mining at the Calumet and Hecla 

 mine had gone down 4000 feet. It is probable that the deep-seated source 

 was a region of veins in Archaean rocks along the line of the fissure or 

 fissures holding chalcopyrite, the most common of copper ores. 



Another example is that of the remarkable Comstock lode, Nevada, along 

 a faulted fissure — now a deserted mining region. The igneous rock at the 

 broad vein is of the basaltic type, and intersects a region of andesyte of 

 Tertiary age. The ore deposit extends along the contact of the igneous rock 

 with those it intersects. The gangue is mainly quartz. The ore is largely 

 silver sulphides with some native silver and native gold, the last nearly half 

 the value of the products. Hot vapors ascend the opening, and during the 

 working it made the cooling of the air with ice necessary in order to reach 

 the lower depths ; and finally the heat caused the desertion of the mine. By 

 means of the vapors, the diabase and other adjoining rocks had become 

 deeply decomposed to clay. The total yield up to July, 1880, was over 306 

 millions of dollars. (King, 1870 ; Becker, 1882 ; Hague and Iddings, 1885.) 



In other related veins, the rocks cut through by the upper part of the 

 fissure vary in porosity and in other ways ; and some of the beds become 

 impregnated with ores, while others receive little or none. Such impregna- 

 tions are occasionally found where no igneous rock by which they could 

 have been produced is in sight. The following sections, illustrating a case 



314. 315. 



of this kind, are from a report made in 1879, by Rothwell and Crouch, on a 

 district on Virgin Eiver, in Utah, 250 miles south of Salt Lake. The 

 formation containing the ore-beds (o) is probably Cretaceous (see Gilbert's 



