225 
i8j9‘] T/te Heat of the Comstock Mines . 
cold, and what may be called a burnt out, layer of rocks, 
extending for 1000 feet below the surface, a zone of hot rock 
still in adtive decomposition, which has been found to exist 
for a depth of about 1500 feet more, and no doubt extends 
thousands of feet further, and, finally, a mass of cold rock 
at a great depth, which has not yet begun to decompose. 
The peculiar bands of hot and cold rocks which the author 
describes are simply layers of rock in which decomposition 
has been delayed or hastened. When the texture of a rock 
is such that it resists decomposition longer than other layers 
in its neighbourhood, it will be at its maximum temperature 
long after its fellows have passed theirs and cooled down, 
and this the author conceives to be the situation of the hot 
bands. They are individual layers of rock undergoing 
delayed decomposition. 
On the other hand, when a rock is peculiarly susceptible 
to the adtion of the air and water, its alteration will proceed 
more actively than that of the surrounding rock. It will, 
therefore, pass its maximum temperature sooner, and be 
cooled down by the time that its neighbours begin to be at 
their hottest. This is the state of the cold bands. These’ 
bands, in fadt, offer at several places in the mines examples 
in miniature of the adtion that is going on upon a grand 
scale throughout the whole system of rocks. 
All the known fadts strengthen the supposition which is 
advanced in this report, that the heat in the mines issubjedt 
to a steady and moderate increase as their depth is increased, 
this comparatively regular progression being broken by the 
passage through belts of rock heated above the average of 
the “ country.” 
The author then considers the relation of temperature to 
-depth, and concludes his paper by remarking that the 
Comstock mines offer a greater promise of discovery in this 
matter of rock temperature than any other he is acquainted 
with. The extraordinary rapidity with which their opera- 
tions are prosecuted, the extent of the works, and the fadt 
that they open to inspedtion a great eruptive mineral lode 
thoroughly for two miles in length, and partially for many 
thousand feet more, give them unusual value as a field for 
investigation. They not only follow an eruptive dyke 
throughout its course, but they also explore a parallel system 
of eruptive rocks by crosscuts, which are often from 300 to 
500 feet long, and sometimes stretch out to 1000 feet and 
more. 
They are also certain to be opened to much greater depths 
than now, and with a rapidity that will no doubt make them 
VOL. ix. (n.s.) q 
