Geography and Travels

1880 American Naturalist  
x88o.] Geography and Travels. 385 Prof. John A. Church has recently written a book on "The Formation and History of the Comstock Lode." His account of Comstock geology differs essentially from this. He concurs with his predecessors in regard to the position and order of the rocks and the presence of a dike under the lode; but he gives to the rocks and to the lode itself a different history. He finds that the diorite and propylite are both stratified, and their strata are approximately
more » ... oximately conformable. They were laid down in the horizontal position, and have been elevated into a mountain range by the ordinary operation of pressure and folding. The dikes of andesite have not broken through cracks opened across the other rocks, but are bedded, interposed between the strata of diorite and propylite. The openings between these strata were not originally so thick as the quartz seams now are. At first they were the merest partings between two layers of the propylite; and in accounting for the development of these insignificant crevices to ore-bodies two and three hundred feet thick, Mr. Church advances one of the most important observations of his book. He takes the bold ground that the Comstock is not a true fissure vein; but that it has been formed by the process of substituting quartz for the propylite in certain localities, which were prepared for the process in a way described by him. The lode lies on one of the andesite dikes, and the metal-bearing water rose up the face of this dike, and penetrated the propylite strata whenever they had been opened. This water was siliceous, and attacked the propylite rock, dissolving it and depositing silica in its place. Each layer of pro. pylite was attacked on two sides, until finally the whole layer was removed, and the two seams of quartz met. The layers of propylite are of all thicknesses, from a few feet up to many yards, and when a number of them were involved in the process of substitution, some would be completely removed, when others were only half dissolved away. If the process of substitution stopped at this stage, the result would be a mass of quartz enclosing streaks and layers of propylite, just as the structure is found to exist at the edges of the quartz bodies.-Engineering and Mining 7ournzal. GEOGRAPHY AND TRAVELS.' AsIA.-Col.
doi:10.1086/272565 fatcat:u6uvbdswajcuna7v7houwnpbia