GEOTHERMAL GRADIENT DETERMINATIONS 35 
straight line, sloping slowly downwards, showing the gradual cooling—of the 
order of a degree or two a month—which takes place as heat is conducted 
from the rock into the slightly cooler drift. A slight extrapolation of the curve 
gives the initial (virgin) rock temperature with an accuracy believed to be 
of the order of 1/10°C. To make sure that drilling and blasting effects are 
really negligible, as any calculations which can be carried out would indicate, 
an occasional 14 foot hole has been run in at the bottom of a short cross-cut 
such as occasionally occurs in drifting. In such a hole the tests made so far in- 
dicate a steady temperature for weeks and months and go far towards remov- 
ing all doubt in one’s mind as to any transient effects. The agreement which 
has been found so far between measurements in such deep holes and the 
standard 7 foot ones is good, but much more work remains to be done along 
this line before complete confidence can be placed in the results, however 
satisfactory they appear to be as a whole. 
RESULTS 
The results to date are shown in Fig. 2. The points determined in the 
present work are seen to lie very well on a smooth curve which is almost a 
straight line. When this is continued to the surface level, making such use 
as seems justifiable of the Agassiz measurements, and especially of the point* 
indicating average surface temperature, we have the geothermal curve for 
this particular spot—insofar as it can be considered to be established by 
these measurements; it is unfortunate that there have been no recent op- 
portunities for reliable measurements in the upper mine levels. The absence 
of local heat sources in this region, as well as general geological considera- 
tions, would lead one to believe that this curve will probably be found to be 
typical of this region, but measurements in other mines will be made as op- 
portunities offer. A comparison with all the Van Orstrand drill-hole data for 
this locality must also be made. It may be noted in passing that the average 
gradient found here of 1°C in 59.5 meters is less than half as steep as that 
taken by Kelvin as the average for the whole earth, viz., 1°C in 27.76 meters. 
Theoretical interpretation of results 
A preliminary attempt has been made at the interpretation of this curve 
in terms of the previous thermal history of this region, involving particularly 
the time since the last ice age. In order to do this it is first necessary to de- 
termine the thermal diffusivity (thermal conductivity divided by the prod- 
uct of specific heat and density) of the rock material. B. O. Peirce® made a 
number of careful tests of the conductivity of specimens of rock from the 
Calumet and Hecla mine many years ago, but unfortunately several assump- 
tions are involved when it comes to getting diffusivities f:om these results. 
Accordingly, a few tests on specimens brought from two regions in the mine 
‘ This is the point taken by Lane (reference 2, p. 703-705) as the average surface tempera- 
ture at Calumet. It was arrived at in several ways, all of which give nearly the same result and 
is believed to be a very good average. 
5 B. O. Peirce, Proc. Am. Acad. of Arts and Sci. 38, 652 (1903). 
199 
