lo UNDERGROUND TEMPERATURE. 
The source of this heat is attributed to chemical action now main- 
tained in the eruptive rocks. But it is remarkable that the heated 
rocks occur only in belts with cold masses between them. 
Professor John A. Church^ supposes the existence of a cold, 
and what may be termed a burnt-out layer of rocks, extending for 
looo feet below the surface, and a zone of hot rock still in active 
decomposition, which has been found to exist for a depth of about 
1500 feet more, and no doubt, he thinks, extends thousands of feet 
further, and finally, a mass of cold rock at a great depth, which has 
not yet begun to decompose. The author also refers to one of 
the hottest belts, being a quartz seam, which appears to be entirely 
in the dorite ; and though he attributes the heat to chemical action 
in the eruptive rocks, he states that it is not a combustion, for the 
oxidizable constituents are little altered. Now it is difficult to 
imagine chemical decomposition going on in any rocks without 
the oxidizable minerals being affected, especially at a temperature 
of 130° F. 
It would be rash to express any decided opinion on the source 
of the great heat in the Comstock mines, from simply reading the 
paper referred to, but the fact of the oxidizable minerals not being 
affected seems to me to be a strong argument against Prof. A. 
Church’s theory. 
The fact of a quartz vein in the dorite being one of the hottest 
parts suggests another supposition, namely, may not the hot belts 
be fissures, through which hot water was, at one time, ejected, but 
which, in course of time, have been closed up, chiefly with silica 
deposited by the water, but through which heat may still be 
transmitted from below. As an instance of this, I may mention 
the Great Geyser,’’ which has deposited silica several feet thick 
in a crevice. 
With regard to thermal springs and geysers, it is difficult to 
understand the temperature being maintained by chemical action 
1 Monthly Journal of Science, March, 1879, p. 224. 
