SUMMARY. 463 



Sulphur Bank. — TIiG general geology of the Sulphur Bank is indicated in 

 the notes on Clear Lake. The bank itself is a small basalt area, through 

 which hot solfataric springs reach the surface, owing their lieat to the vol- 

 canic action of which the lava eruption was an earlier manifestation. The 

 springs contain much sulphydric acid, which, oxidizing more or less fully at 

 and near the surface, has yielded native sulphur and sulphuric acid. Tlie 

 latter has attacked the basalt in part, extracting the basis and leaving a mass 

 of more or less pure silica, in which rounded nodules of undecomposed rock 

 remain. The rounded form of these nuclei is certainly due to the more 

 rapid corrosion of the edges and corners of the basalt blocks, not to any 

 structural peculiarity of the rock. The lava is bleached to an average 

 depth of about twenty feet. 



In the lower portion of the decomposed la3'er of rock the sulphur is 

 mixed with cinnabar. Near the bottom of this rock layer the sulphur dis- 

 ajipears and the ore is richer, while the most extensive bodies arc found at 

 depths beyond the limits of the action of acid. The ores at one i)ortion of 

 the ground continued down for several hundred feet into the unilerlying 

 recent lake beds and the metamorphic sandstones. Quartz, chalcedony, 

 calcite, pyrite, and marcasite are the usual gangue minerals, but many other 

 minerals are found in small cpiantities. The marcasite contains minute quan- 

 tities of gold and copper. Bituminous matter is widely disseminated. The 

 ore has been deposited exclusively in cavities, and not by substitution. The 

 ore of the lower workings is exactly like that of most other quicksilver mines. 



The gases escaping from the waters are carbon dioxide, hydrogen 

 sulphide, sulphur dioxide, and marsh gas. The waters contain cliietiy car- 

 bonates, borates, and chlorides of sodium, potassium, and ammonium; but 

 alkaline sulphides are also present. At the ordinary pressure the water 

 does not dissolve cinnabar, on account of the presence of ammonia, but I 

 have proved that at somewhat higher pressures it would effect solution. It 

 is beyond question that the cinnabar has been deposited from waters of 

 almost exactly the same composition as those now issuing from the mine 

 and that the formation of ore is still in progress. Depositi(?n of the ore 

 seems to have been effected chiefly by relief of temperature and pressure 

 in the presence of ammonia, not by acidification of the solutions. 



