1884.] 199 [Wadsworth 



pre-tertiary and tertiary andesites. 1 The position then taken has 

 been fully confirmed by Dr. G. F. Becker 2 , who unjustly appropri- 

 ates to himself the credit of this discovery, and is supported in 

 this appropriation by his colleague, Arnold Hague. 



Dr. Becker further states that every rock in the district has been 

 taken for p ropy lite when decomposed. 



The above mentioned changes or alterations in rocks of the 

 same composition appear to be largely dependent upon the action 

 of infiltrating waters, and their rapidity seems proportionate to the 

 temperature. These alterations appear to consist in general, of 

 molecular transferences or chemical reactions in the rock mass as a 

 whole, and are not confined to special minerals ; hence has resulted 

 the failure of theories of mineral pseudomorphism to explain rock 

 metamorphism or alteration — the pseudomorphic changes in the 

 rock mass being but the resulting accident of the greater and more 

 general metamorphosis. In the process of alteration the origina 

 glass of the rock is broken up, forming various minerals according 

 to its composition, while the original crystallized minerals are 

 changed to a greater or less degree ; the whole resulting in the 

 formation of quartz, various ores, anhydrous and hydrous silicates, 

 carbonates, etc. In the course of these changes there is everywhere 

 seen a tendency to localize these secondary products, especially 

 the ores, which results in the removal of material of one kind 

 and the deposition of another in its place, or in the filling of fis- 

 sures and cavities in the rock. It is not uncommon to find minute 

 veins in rocks, which, under the microscope, show variation in 

 their filling material as they pass through different minerals. That 

 ■which has now been described as taking place in one rock takes 

 place in all, and frequently with various interchanges and reactions 

 between the different associated rocks. If instead of minute fis- 

 sures to be filled and the alterations to be observed under the 

 microscope, we gradually pass to deposits visible to the unaided 

 eye, and to the joint or fissure planes affecting large masses of 

 rock, to cavities or to any condition of rock structure that will 

 admit of deposition of mineral matter, then whether we have ore 

 deposits or not seems to depend upon the activity of the altera- 

 tion and upon the amount and kind of matter stored. It is well 

 known that valuable ore deposits are more apt to occur in regions of 



. iBull. Mus. Comp. Zool., 1879, Vol. V, 281, 285, 28G. 



2 Geology of the Comstock Lode and Washoe Districts. 1882, pp. 12-150. 



