ALTERATION' OF THE COUNTKY-ROCK. 235 



ankerite by Professor Silliinan arc in reality, as indicated hy IT. \\\ Fair- 

 banks,^ a mixture of varying composition, ranging: from calcite to maj:;- 

 nesite, and often containinij: considerable iron. Magnesic carbonate, on 

 the whole, predominates. The mineral mariposite is, as Silliman observes, 

 onlN' associated with magnesian and chloritic rocks. Fairbanks t states 

 that it is particularly cliaracteristic of the mother-lode. This is not 

 correct. It is, however, eminently cliaracteristic of all quartz- veins in or 

 at the contact of serpentine, though occasionall}^ occurring in very small 

 quantities in diabase and other basic rocks. Tlie writer lias noticed the 

 same characteristic mixture of carbonates and mariposite from a great 

 many places in the gold-belt besides the mother-lode; thus, for instance, 

 at the Phcenix and Red ('hief mines in Sierra county, and also near 

 Washington, Nevada county. It appears at the mother-lode wherever 

 that great quartz-vein breaks through serpentine. Quartz mountain, 

 Tuolumne county, is an excellent place to study it. 



Along the mother-lode the altered serpentine has been variously inter- 

 preted. Whitney inclined to the belief that the vein represented a 

 stratum of silicitied dolomite, a theory that has not been supported by 

 more detailed investigation. Fairbanks, who some years ago carefully 

 examined the mother-lode. + considered it at first as vein-matter deposited 

 in open fissures, but regarded it subsequently (as the needed, once open 

 space would manifestly have been too large, in places several hundred 

 feet) as an altered, coarsely crystalline basic rock. The latter theory, 

 while nearer the truth, is unnecessary. A careful investigation will not 

 fail to disclose the fact that the mixture of carbonates and mariposite is 

 notliing but an altered serpentine, and al)undant transitions may be 

 found to j)rove this. A locality showing this ])lainly is the App mine at 

 Quartz mountain, Tuolumne county. This conversion is not astonishing 

 wlien the facility is considered with which the serpentine is decomposed 

 by carl)onated waters into magnesite and chalcedonic quartz. Experi- 

 ments by C. Doelter§ show that while at ordinary temperature and 

 pre.-^sure water containing carbon-dioxide will, with simultaneous decom- 

 position and formation of carbonates, dissolve 0.3 per cent orthoclase and 

 0.5 per cent oligoclase, serpentine will be dissolved at the rate of 1.24 

 per cent. 



The large bodies of decomposed rock referred to on page 22G as con- 

 taining impregnations of auriferous pyrites and rarely free gold are in 

 many respects interesting. In the ferruginous outcrops the iron-pyrites 

 is usually converted into ferric hydroxide and the gold set free ; the whole 



•Tenth Ann. Kep. Stale Miiu-iftloKist, p. 85. 



t Loc. cit. 



I Loc. fit. 



g Allgemine rhemischc goologio, Leipzig, 1890, p. 190. 



