28 GEOLOGY OF THE COMSTOCK LODE. 



found; and herein is another point of resemWance to the old diorite-por- 

 phyries. 



"All these diffei-ences between propylitic and andesitic liornblende also 

 extend to both of the quartziferous members, quartz-propylite and dacite." 



On page 1 1 7 Professor Zirkel states that the quai-tz of quartz-propylite 

 contains fluid inclusions, and "behaves exactly like that of the ante-Tertiary 

 dioritic porphyries, and differently from that of all other Tertiary quartz- 

 iferous rocks, dacites and rhyolites, which only contain glass inclusions." 



Church's memoir, — lu 1877 Mr. J. A. Cliurcli mado an examination of the 

 CoMSTOCK,' as a member of the United States Surveys West of the One 

 Hundredth Meridian, under Captain Wheeler. Mr. Church accepted the 

 lithology of his predecessors with some modifications a little difficult to fol- 

 low, but though he mentions slides of the rocks, describes none. His 

 memoir contains a number of ingenious hypotheses which would possess 

 great importance if sufficient evidence in their favor could be adduced. 



Lithology. — Mr. Church appears to use the terms porphyrite and propylite 

 interchangeably for all strikingl}' jJorphyritic rocks of light color.- Rocks 

 of dark color, whether from the presence of abundant hornblende or from the 

 transparency of the feldspars, he appears to have regarded as andesite,^ and 

 asserts that it is quite safe to put the minimum number of north and south 

 dikes of this rock at between twenty-five and fifty. Besides the masses 

 which had hitherto been regarded as trachyte, he determined the rocks 

 about the new Yellow Jacket shaft, and at other points, as remnants of the 

 trachyte eruption. This leads to the supposition that he employed the term 

 merely to designate soft, rough, light-colored, porphvritic rocks. In Mr. 

 Church's opinion the diorite, propylite, and probably the andesite, were laid 

 down in thin regular layers, which he compares to those of sedimentary 

 rocks. This he considei's proved by the sheeted character of the rocks in 

 the east and west countrj^. 



'The CoMSTOCK Lode, its formation and history. By John A. Church, E. M., Ph; D., member 

 of tho American Institute of Mining Engineers, mining engineer. Illustrated by six plates and thir- 

 teen figures. New York : John Wiley & Sons. 1879. 



■^L. 0., p. 40 to 4-2 and 52. 



'L. c., pp. 37 and 47. The McKibbeu tunnel shows only diorites and quartz. 



