36 MINING INDUSTET. 



of that establishment were obtained. It is of the sanidin variety, partly a 

 heavy, dense rock, from which the foundations themselves were made, partly a 

 pinkish breccia, very loose in texture, and partly of a grayish, shaly sandstone 

 mass, which is unlike any of the trachyte forms observed in the Great Basin. 

 This is a singular segregation of fine, sandy material which, shading into the 

 other rock, has lost every trace of crystalline structure, and, even under the 

 microscope, appears to be an aggregation of rounded particles, but, as it 

 approaches the main trachyte, gradually becomes more and more crystalline, 

 and passes imperceptibly into the ordinary variety without any apparent junc- 

 tion line. Certain specimens cannot be separated from metamorphic sand- 

 stones. Toward the middle, the mass has an open, shaly structure, and con- 

 stantly slacks under the influence of the atmosphere, like the shales of the 

 Cretaceous period. 



Traversing the whole trachyte formation are a series of parallel north 

 and south jointing planes, which, like all the others of the region, dip at a high 

 angle to the east. 



The trachytes are easily distinguished from the other rocks of the country 

 by their porous, lava-like texture and the loose porphyritic mode of occur- 

 rence. Their limits are always easily defined upon the propylite, which they 

 have overflowed. It is evident that the greater part of the erosion has taken 

 place since the trachyte period, for they share with all the earlier rocks the 

 abrupt sulcated surface. 



Basalt. — In the southwest corner of the maj) there are five patches of 

 black, which represent all that is left of a great sheet of basalt, which, at the 

 close of the volcanic period, poured out over the metamorphic rocks and quartz- 

 propyhtes. From their mode of occurrence and characteristic jointing, they 

 may be easily distinguished from the black metamorphic rocks of the same 

 region; the latter are traversed by innumerable thin seams of carbonate 

 of lime ; the former are impregnated with considerable quantities of oKvine. 

 Traces of parallel structure are discernible in the metamorphics, while the 

 basalts are clearly parts of a single flow. 



This basaltic table is a feeble representation of a vast ejection which 

 brought to a close the era of volcanic activity. 



