206 GEOLOGY OF THE COM8TOCK LODE. 



andesite, there can be little doubt that it is the youngest of all. Its relations to 

 the andesites have been observed in a great number of localities in the western 

 United States, and it has always been found to succeed them. This general 

 evidence is strengthened in the present case by the extreme freshness of the 

 olivine, which even under the microscope often shows no trace of decom- 

 position. As olivine is the most readily decomposed of all the lithologically 

 important minerals, this fact is evidence that the basalt is very recent. 



Period of soifatarism. — Tho gcologists wlio havc studicd the CoMSTOCK have 

 always sought to connect the solfataric action, which is so important a feat- 

 ure of the District, with one or other of the volcanic eruptions. Since the 

 augite-andesite and the rocks which preceded it are deeply altered by soi- 

 fatarism; and even portions of the younger hornblende-andesite are also thus 

 affected, the general decomposition cannot be placed earlier than the erup- 

 tion of the last-mentioned rock. Portions of an eruptive rock may be im- 

 mediately decomposed by the emanations accompanying its ejection, but 

 before an extensive area can be decomposed throughout, it must probably 

 cool and be shattered by mechanical action sufficiently to admit a some- 

 what free penetration of active solutions. If the solfataric action is due to 

 one of the eruptions, it must then be either to that of the younger horn- 

 blende-andesite or to that of the basalt. But direct evidence of such a 

 connection is wanting. The focal line of soifatarism is at or close to the 

 Lode. The younger hornblende-andesite area shows no trace of it except 

 where it approaches the vein ; and, as has been mentioned, the basalt shows 

 no effects of solfataric decomposition. It is also somewhat difficult to under- 

 stand how an eruption can produce extraordinarily intense solfataric action 

 at a locality somewhat remote from the vent of its own fluid ejecta, and not 

 also at or close to that vent ; though I by no means deny the possibility 

 of such a coincidence. 



While, however, the solfataric action appears to me, beyond question, 

 one of the series of volcanic events of which the history of the District 

 is so full, it does not seem to be necessarily connected immediately with an 

 eruption of lava There are mud volcanoes; and solfataras are often active 

 at periods of time remote from those of eruptions in their neighborhood, 

 and tliough the emission of heated waters frequently attends igneous erup- 



