26 GEOLOGY OF THE COMSTOCK LODE. 



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

 charges of metal-bearing quartz. The latest flows of andesite poured out 

 over the decomjjosed propylite; and since they are themselves unaltered, 

 their appearance marks the period when solfataric action over wide areas 

 had ceased. While it no longer maintained its energy through the broad 

 zone of propylite, it still continued intensely active within the chambers of 

 the Comstock Lode. Metallic contents were introduced into the quartz, 

 the clay seams were formed by a rapid decomposition of the neighboring 

 propylite materials, the horses reduced to a spongy, semi-plastic condition, 

 and at last the final solidification of the quartz took place. Outside the 

 vein two events of geological interest have occurred: first, the period of 

 trachyte eruptions, when from the ruptures of the crust, parallel to the 

 Comstock Lode, vast volumes of sanidin-trachyte overflowed the country; 

 and, secondly, the less powerful but still important outpouring of basaltic 

 rock, which marked the close of the volcanic era. Within the vein, and 

 probably caused by one or both of these latter volcanic disturbances, a 

 pressure has been exerted which has crushed and ground the masses of 

 quartz into minute fragments. It is interesting to obsei ve that while this 

 force was great enough to crush quartz masses 150 feet in breadth into mere 

 angular pebbles, the disturbances were insufficient to cause any actual 

 faulting of importance. Both within and without the vein the solfataras 

 gradually came to a close. The heated currents of water which even yet 

 ascend into the lower levels of the mines, are evidence that at no very 

 great depth a considerable temperature is still maintained; but this is only 

 a faint relic of a once intense action." 



zirkeis report. — lu \S7f) Prof Fcrdiuaud Zirkel made a macroscopical and 

 microscopical examination of the lithological collections of the Exploration 

 of the Fortieth Parallel.^ Among the slides which he described are thirty- 

 three from the Washoe district He confirmed the independence of horn- 

 blende-propylite and quartz-propylite as lithological species, regarded most 

 of the quartzose rock as dacite, corrected the determination of the granu- 

 lar diorite (which had been considered as syenite), and added augite-ande- 



» Exploration of the Fortieth Parallel, Vol. VI. 



