THE COMSTOCK LODE. 95 



encouragement in a financial point of view; but the absence of bad prospects 

 is in itself an encouraging feature, and there is certainly nothing in the present 

 lowest developments of the lode which will at all warrant the belief that the 

 metallic yield must necessarily cease. While it seems improbable, from the 

 configuration of the ground and the relations of the fissures, that there can 

 ever again be such a magnificent expansion of bonanzas, yet there is no strong 

 reason why a total cessation of the silver minerals should be expected. The 

 examples of other great veins, all that we know of the mode of deposition 

 and chemical transmutations of silver ore, rather favor the idea of a continued 

 silver occurrence in depth. But the lode is in many respects unique, and it 

 may be considered as established that only actual exploration can determine 

 the important question of the future. 



Resume. — The results of this long and perhaps tedious investigation may 

 be summed up in the following statement of conclusions: The ancient Virginia 

 Range, prior to the Tertiary period, was composed of sedimentary beds of the 

 great Cordillera system, which, in the late Jurassic epoch, had been folded up, . 

 forming one of the corrugations of that immense mountain structure which 

 covers the western front of our continent. Accompanying this upheaval 

 were outpourings of granite and syenite. The erosion which foRowed this 

 mountain period escarped the ancient rocks, and modeled the eastern front 

 of Mount Davidson into a comparatively smooth surface, whose average 

 angle of slope sank to the east at about 40°. In the late Tertiary, at the 

 time of the volcanic era, the Virginia Range shared in the dynamical convul- 

 sions which gave vent to successive volcanic outflows of immense volume and 

 very remarkable character. The first and, so far as the Comstock lode is con- 

 cerned, the most important, was of propylite, or trachytic greenstone, which 

 deluged the range from summit to base, covering large portions of its ancient 

 surface, and leaving here and there isolated masses, which rose like islands 

 above the wide fields of volcanic rock. Subsequently followed the period of 

 the andesites which, at their commencement, iu the form of a thin intrusive 

 dike, penetrated a new-formed fissure on the contact plane of the ancient syenite 

 and the propylite. This earlier andesite period gave birth to the solfataras, 

 which, bursting from a hundred vents, rapidly decomposed the surrounding 

 rocks, and gradually filled the fissures of the Comstock with their remarkable 



