498 DESCRIPTIVE GEOLOGY. 



galena veins has been shown, but in none does it occur in such large quan- 

 tities or in so well-crystallized specimens as in the Tecoma District, Ombe 

 Mountains. The molybdate of lead frequently forms so high a percentage 

 of the ore as to interfere seriously with its treatment in the ordinary lead- 

 furnaces, rendering a modification of the methods employed very desirable. 

 The crystallized wulfenite from the Tecoma Mine occurs in large masses, 

 the faces of individual crystals having been observed from an inch to an 

 inch and one-half in length. They possess a resinous lustre, a lemon-yellow 

 color, and are frequently transparent and exceedingly brittle. In size and 

 brilliancy, the finest specimens far surpass the famous wulfenite crystals 

 from the limestones of Bleiberg in Carinthia. - Associated with the wulfenite, 

 adhering to the broad tabular faces, may occasionally be seen well-developed 

 crystals of cerusite and anglesite (carbonate and sulphate of lead). 



At another locality in the mountains occurs a quantity of bluish-green 

 and brown chrysocolla (silicate of copper) associated with some oxide and 

 • carbonates of copper, but of no economic importance. 



On the eastern slopes of the Ombe Mountains, about 10 miles south of 

 the railroad, there are exposed, in the deeper cuts and ravines, sections of 

 upturned sedimentary beds of Tertiary age, from which no fossils were 

 obtained, but which, from their close lithological resemblance to beds of 

 this age, as developed in Peoquop Pass and Elko Valley, have been con- 

 sidered to represent the Green River Eocene. They dip at an angle of 45° 

 to the eastward, with a strike of north true. They consist mostly of white, 

 thinly-bedded calcareous and siliceous shales, remarkably fissile and fre- 

 quently bituminous, carrying several thin seams, from 1 to 2 feet in thick- 

 ness, of light coal. This coal, though black and lustrous, and apparently 

 free from earthy matter, is, like that of Elko, very light, crumbles on 

 exposure to the air, and will probably prove of little economic value. 



Later Tertiary beds, lying approximately horizontal, probably of Plio- 

 cene age, occur at the northern end of the mountains, cropping out in a few 

 localities, where exposed from beneath the Quaternary deposits. They con- 

 sist, for the greater part, of fine siliceous rhyolitic material, interstratified 

 with occasional fine sands. In color, they are light lavender and grayish- 

 white, showing an even texture, with but little variation in composition. 



