LINDGBEN.] SILICIFIC ATION. 645 



altered granite immediately adjoining this piece gave only a trace of 

 gold and silver. Under the microscope the altered granite is seen to 

 contain large, partly crushed quartz grains. The feldspar, which is 

 partly orthoclase, partly a soda-lime-feldspar, is filled with sericite 

 foils, chiefly developing on the cleavage planes, and a few grains are 

 almost totally replaced by this material. A few larger muscovite foils 

 probably represent the altered biotite. No calcite is present. 



The diorite-porphyrites and quartz-diorite-porphyrites of the Gold 

 Hill and Pioneer mine at Quartzburg show a similar alteration, chiefly 

 consisting in a conversion of the hornblende and biotite to muscovite 

 foils, calcite, and perhaps also pyrite, while there is a great develop- 

 ment of fine-felted sericite in 'the feldspar phenocrysts and in the 

 groundmass. The abundant pyrite is in sharp cubes, usually lined 

 with sericite foils. Aggregates of secondary quartz also develop in 

 places. 



Silicification. Mr. S. F. Emmons, in his studies of the mineral depos- 

 its of Colorado and other parts of the Rocky Mountains, has admirably 

 and with deserved emphasis brought out the fact that replacement is 

 a process to which many deposits owe their origin, and that it plays an 

 important part in almost all deposits caused by mineral-bearing waters. 

 Carried away with the importance and interest of these results, many 

 geologists and mining engineers have, however, extended the theory 

 of replacement beyond its proper bounds, and speak of every vein- 

 filling and even of veins of solid white quartz as products of replace- 

 ment. Against this view a strong protest should be entered. In this 

 connection it may be of interest to consider, briefly, the processes by 

 which silicification may be produced. 



In the case of ores consisting of carbonates there may often be some 

 difficulty in deciding what is filling and what is replaced country 

 rock, for carbon dioxide and alkaline carbonates are very strong sol- 

 vents, attacking easily nearly every one of the rock-forming minerals 

 and forming pseudomorphs after them. The carbonates may replace 

 a rock completely, wholly changing both composition and structure. 

 As an instance may be cited the coarse-grained mixture of carbonates 

 and mariposite (fuchsite a chromium mica) resulting from the 

 replacement of the serpentine along the Mother lode of California. 



As to quartz, the conditions are wholly different. A solution of 

 silica is comparatively inert and does not easily attack any of the 

 rock-forming silicates. Silicification may take place by two greatly 

 differing processes: (1) Cementation, or filling of the interstices of 

 porous or shattered rocks by quartz deposited from solutions; (2) 

 metasomatic silicification, or a substitution of silica for other miner- 

 als, the silica either being produced by the alteration of the original 

 minerals or deposited pari passu with the dissolving of the original 

 mineral by active reagents in the waters causing the metasomatic 

 action. 



