PREVIOUS INVESTIGATIONS. 25 



between this rock and propylite, and it will not be at all surprising if it 

 should finally prove to be chemically identical and, in reality, only a different 

 form." The rock determined as quartz-porphyry by Baron v. Richthofen, 

 Mr. King regarded as quartz-propylite. 



In discussing the relation of Mount Davidson to the vein, Mr. King 

 calls especial attention to the agreement between the contours of the west 

 wall of the Lode and those of the exposed face of the range. He considers 

 this similarity as evidence that the wall is merely a continuation of the face 

 of Mount Davidson, and that the syenite has undergone very little erosion 

 since the opening of the fissure. It is not necessary to summarize Mr. 

 King's very graphic description of the structure of the Comstocic Lode in 

 detail here, as I shall be obliged to use it in my own account of the vein, 

 the portion which he examined having long been inaccessible. 

 Mr. King closes his memoir with the following summary : 

 King's summary. — " Thc ancicut Virginia Range, prior to the Tertiar}^ 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 corruga- 

 tions 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 followed 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 convulsions which gave vent to suc- 

 cessive volcanic outflows of immense volume and very remarkable character. 

 The first and, so far as the Comstock Lode is concerned, the most import- 

 ant 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, in the form of a thin intrusive dike, pene- 

 trated a new-formed fissure on the contact plane of the ancient syenite and 

 the propylite. This eariier andesite period gave birth to the soLfataras, 

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



