516 GEOLOGY AND MINING INDUSTRY OF LEADVILLE. 



longer constituted an essential part of the mining industry of Leadville, nor 

 could detailed studies be made of them, yet some suggestions are afforded 

 by the consideration of the general geological conditions of the region that 

 are not without value. 



The richest portions of the California gulch diggings are said to have 

 been, first, in the bend below Oro, at the mouth of Nugget gulch ; next, in 

 the bend at the La Plata mine; and then in that below Graham gulch. The 

 first was exceptionally rich, and in the narrow bed of the gulch at Oro a 

 gold-bearing cement, containing hydrated oxide of iron, was found below 

 the gravel, which yielded one ounce of gold to the ton. The gulch gold 

 was worth $17 to Si 9 per ounce, while that from the mines was worth only 

 $15. From its propinquity the Printer Boy Porphyry, known to contain 

 actual gold veins, suggests itself as the source of these rich gravels, with 

 the oxide of iron resulting from the decomposition of pyrite in the Pyritif- 

 erous Porphyry as a cementing material. Moreover, the Weber sandstones, 

 at the head of the gulch, have been found to cany gold veins, and from 

 their abrasion also gold-bearing gravels would have been earned down 

 the gulch. These probable sources of gold seem, however, inadequate to 

 account for the greater relative richness of California gulch over its neigh- 

 bors, since both Evans and Iowa gulches contain gold-bearing veins and 

 the amount of sandstone and porphyry debris that has been carried away 

 through either of their beds must have been far greater than that swept 

 through California gulch. 



It seems very doubtful whether in general all, or .even the greater part, 

 of the gold contained in placer gravels is derived from the abrasion of actual 

 gold veins. Traces of gold may be found in a very large proportion of the 

 massive rocks which form the earth's crust. Gold veins are concentrations 

 of this mineral in sufficient quantity to attract attention and yield a profit 

 to the labor of man; but doubtless there are a vast amount of smaller con- 

 centrations which may escape his notice. As the rock disintegrates and is 

 worn away by atmospheric agencies the gold from these smaller deposits, 

 as well as from the larger, is set free from its inclosing rock and subjected 

 to the concentrating action of mountain streams. Placer deposits are the 

 results of nature's vast sluicing processes. To bring them into the condition 



