REPORT ON SUBTERRANEAN TEMPERATURE. 317 
The upper line represents the surface of a given district, 
and the diagonal one the line of junction of granite and killas. 
The dotted lines show the mean intervals at which, according 
to the first Table, there appears to be a progressive augment- 
ation of 10° Fahr.; but the tortuous line x y might more 
properly indicate the very irregular depths at which a given 
amount of temperature exists, even in the same neighbour- 
hood. 
The isothermal lines are represented as having a small 
inclination downwards as they pass from the killas into the 
granite, to illustrate the inferior temperature of the latter. 
The amount of this difference is undoubtedly very variable in 
different localities, and sometimes little or nothing. I have, in 
my earlier papers on subterranean temperature, noticed the 
- fact, although I did not ascertain the extent of the difference *, 
nor have I considered it so high, upon the whole, as Mr. W. J, 
Henwood has done; but he has investigated this point much 
more fully than I have, and made numerous experiments in 
reference to it. 
The intervals between the isothermal lines seem to vary 
much in different places; but I think it will be found to be a 
general fact, that the temperature increases less rapidly in 
descending in proportion to the depth of the stations in the 
mines. If so, the conducting power of the rocks cannot, I 
apprehend, be considered as the immediate or proximate cause 
of these phenomena at the greatest depths hitherto attained : 
nor is it to be supposed, that a depth where the heat is trans- 
mitted through this medium only will ever be reached by 
man, seeing that the temperature at the bottom of some deep 
mines is already almost as great as is compatible with active 
operations. 
I have often suggested, that the differences which are found 
to exist in the increments of temperature in different places 
and strata, are principally caused by the circulation of water 
under the surface; and the tendency of warm water to ascend 
through cooler portions of that fluid is quite consistent with 
_ the fact of the ratio of increase being greater at small than at 
considerable depths. Wherever the facilities are the greatest 
veins, faults or fissures in the strata, and frequently at the 
junction of different rocks, there the subterranean temperature 
for the ascent of these currents, such, for instance, as exist in 
is usually found in excess, or above the mean. Let the points 
_* It seems almost needless to remark that, in comparing the subterranean 
_ temperature in different rocks, reference should be had to the depth of the 
stations at which the observations were made. 
