30 MINING INDUSTET. 



the rhyolites. A fissile, parallel structure further indicates the increase 

 of trachytic tendency. 



Andesite. — The first appearance of a later rock than the propylite is 

 found in the andesite dike lying along the contact plane of Mount Davidson. 

 The earliest andesite probably inaugurated the solfataras. Near the close 

 of their activity, or perhaps subsequently, there were formed three zones 

 of fissures vi^hich traversed the earlier and volcanic formations alike. 

 Through them there penetrated to the surface, and outpoured to a consid- 

 erable extent thin table-like overflows of andesitic rock. The first, or 

 summit zone, consists of a group of conchoidal fissures, following approxi- 

 mately the summit line, and cutting through the syenites of Mount Davidson, 

 as well as the propylites both north and south of it. On the heights above 

 the Ophir grade considerable fields of andesites cover the summit, and pour 

 downward over the propylite to the north of the road. 



Ascending Crown Point ravine two of these dikes are passed, with an 

 intervening distance of a thousand feet of propylite. Their outcrops rise 

 boldly from the surface on either flank of the ravine, and from the average 

 appearance indicate a thickness of a hundred feet. Directly west of the 

 summit of Mount Butler, and at a corresponding point on the Davidson ridge, 

 the same system of andesite dikes are passed. They have overflowed here a 

 wide area, and the debris covers the western slope to a distance of a 

 thousand feet downward. Continuing northward, with a break from Middle 

 Hill to the crest of Ophir Hill, upon the level plateau-like summit, has been 

 ejected a considerable table. 



The second zone, parallel to the first, traverses the American Flat and the 

 plateau in front of Virginia, its outcrops disappearing to the northward under 

 the later outpourings of the sanidin-trachyte. Rather more continuous than 

 the first zone, the outcrops are, however, less expanded, and a subsequent 

 erosion, which has largely worn them away, reveals an extraordinary thinness of 

 flow. Approaching Virginia from the south the first of these andesite fields is 

 found upon a rounded knoll south of Gold Hill, where for an area of ten 

 or fifteen acres the entire surface is of andesite. Thin streams have poured 

 from a narrow fissure downward toward Gold Canon, partly covering what 

 is known as the Graveyard spur. Continuing northward, the sharp conical 



