262 QUICKSILVER DEPOSITS OF THE PACIFIC SLOPE. 



accounted for by supposing that the ore was precipitated by tlie sulphuric 

 acid forming at the surface. The rock is attacked by acid to a depth of only 

 about twenty feet, and the riclier ores are found at lower levels, where no 

 evidence of the presence of unneutralized acid occurs and where the composi- 

 tion of the ore is substantially similar to that in the deep workings. If the oi"e 

 had been precipitated by acidification of the solutions, it would be mainly 

 found in the upper part of the bank or along the under surface of the layer 

 of basalt which has been bleached by acid. This is not the case, and 

 hence, while acidification of the solutions would undoubtedly have thrown 

 the quicksilver down, other causes of precipitation must have been at work, 

 and indeed must have been the chief ones. The fact that sulphuric acid 

 forms at the surface is also insufficient to account for the absence of cinna- 

 bar from the surface, for at Steamboat Springs, where acid forms in the 

 same way as at Sulphur Bank, cinnabar did reach the surface. The forma- 

 tion of sulphuric acid from hydrogen sulphide is not a rapid process, and in 

 springs from which there is a considerable flow of water neutralization by the 

 acid thus formed coukl take place only to a very short distance from the sur- 

 face. The resulting distribution of ore would also be extremely irregular. 



There is indeed no proof that the main period of deposition of ore at 

 Sulphur Bank was contemporaneous with the chief deposition of sulphur 

 and the formation of sulphuric acid. One might rather suppose that when 

 the deposition of ore was progressing most actively the upward flow of 

 solutions and the emission of gases were too vigorous to permit the per- 

 meation of the upper part of the bank by atmospheric oxygen. Little 

 sulphur or sulphuric acid would then form, and only at the surface. As 

 the activity of the springs diminished the permeation of oxygen would 

 increase, and the sulphuric acid slowly formed at the surface would have 

 an opportunity to diffuse through the rock. The sulphur beds may thus 

 not improbably be in the main of later origin than the ore. 



While acidification is insufficient to account for the precipitation of the 

 ore, diminution of pressure and of temperature must certainly have taken 

 place as the solutions rose to the surface. So far as is known, too, these 

 causes may be sufficient to explain the observed effect, but dilution with 

 waters percolating from the lake or from springs may have contributed to 

 the result. 



