8 MINmG mDUSTEY. 



broad Tertiaries which fringe the west base of the Sierra Nevada, any auriferous 

 pebbles have been found; yet, at the same time, the Jurassic origin of these 

 veins is proven from other data, as will be seen from the reports of Professor 

 Whitney. The fact, then, of the absence of the ore-bearing pebbles from 

 the later strata is only negative evidence, and cannot weigh greatly against 

 the probable Jurassic age of very many mineral deposits. So far as we now 

 know, no metallic veins occur in the sedimentary formations of the Tertiary 

 period, east of the Wahsatch Mountains; whereas the remarkable meta- 

 morphism which has occurred during that period in the Coast Ranges, near 

 San Francisco Bay, has developed extraordinary deposits of quicksilver and 

 chromic iron. 



The metallic minerals have, then, in obedience to the prevailing rule of 

 longitudinal structure, arranged themselves in parallel zones, which extend 

 from north to south over great areas. In the history of the Cordillera, two 

 prominent epochs of mountain building have taken place, each accompanied 

 by the ejection of igneous rock with metamorphism and solfataras, and, lastly, 

 with the formation of great numbers of metal-bearing lodes. The lodes belong- 

 ing to the Jurassic age occur chiefly inclosed between strata ranging from that 

 date down into the Devonian, along the planes of deposition or the jointing 

 planes developed by metamorphism and pressure, or, lastly, in the igneous 

 rocks of the granitic family which accompanied the Jurassic disturbance. 



The veins of the second type, which belong to the Tertiary period, are 

 found inclosed either in part or in whole within the volcanic rocks. Their 

 position in that family is among the earlier members, never, so far as is now 

 known, either penetrating the trachyte or basalt. While the greater number 

 of veins probably belong to the first type, some of the most brilliant examples 

 of metal lodes have occurred as members of the second type. 



It is a noteworthy fact that of the series of conformable strata forming 

 the main central mountain masses, not less than six, and perhaps as much as 

 ten thousand feet are the true coal measures of the Carboniferous. This Ex- 

 ploration has demonstrated their continuity over a wide area. They were, 

 however, deposited in the quiet depths of an ocean, and, consequently, have 

 no traces of the beds of coal which characterize them whenever laid down 

 near the water level. The fossils they contain are chiefly marine mollusks, 



