104 FRANK D. ADAMS 
A careful study of these variations has recently been made by 
Konigsberger and Miihlberg. According to these authors: 
It may be assumed that all over the earth in flat countries where the sub- 
soil consists neither of comparatively recent eruptive rocks nor of deposits 
liable to change, there prevails a normal geothermic gradient. All deviations 
from the normal gradient are due to local influences: uneven surface of soil 
(mountains, valleys), the presence of great reservoirs of water (lakes, the sea), 
and heat producing processes in the interior of the earth or incompletely 
cooled eruptive masses.! 
The average of the gradient of 32 borings in flat country in 
rocks which are neither of eruptive origin nor subject to consider- 
able chemical alteration is given by Ko6nigsberger and Mihlberg 
as 1° Centigrade in 32.9 meters. 
The geothermic gradient adopted as shown in Fig. 1 is based 
on this value. 
The view that this temperature gradient continues uniformly 
downward in the earth’s crust has been controverted by Strutt, 
who from investigations into the radio-activity of the rocks of the 
earth’s crust concludes that the temperature rises uniformly to 
t500°C., and then remains constant at that temperature to the 
center of the globe. This conclusion, however, does not affect 
the consideration of the present question, seeing that this tempera- 
ture would not be attained until a depth had been reached which 
is much greater than those whose temperatures are made the 
subject of study in the present paper. 
THE EXPERIMENTAL METHOD EMPLOYED 
Two rocks were employed in this investigation and two series 
of experiments were carried out with each. 
In the first series with each rock, the attempt was made to 
reproduce the conditions of pressure to which rocks are subjected 
at various depths within the earth’s crust, both as to intensity 
and, as far as possible, as to duration; while in the second series 
of experiments the third element of temperature was also intro- 
duced. 
1 J. Konigsberger and Miihlberg, ‘‘On Measurements of the Increase of Tem- 
perature in Bore Holes, etc.” Trans. Inst. of Mining Engineers, London, 1910, 
XXXIX, Pt. IV, 9. 
