APPLICATIONS OF GEOTHERMICS TO GEOLOGY 15 
TABLE I 
TEMPERATURES IN INTERIOR OF EARTH 
Depth, Jeffreys Adams Depth, Jeffreys 
Kilometers Ce SG: Kilometers Ae 
° ° ° 300 2,090 
25 600 560 350 2,290 
50 800 860 400 2,490 
75 940 1,090 45° 2,670 
100 1,080 1,290 500 2,840 
125 1,220 1,470 550° 3,010 
150 I, 360 1,620 600 3,170 
175 1,490 1,760 650 3,330 
200 1,620 1,900 700 3,490 
225 1,750 2,030 
250 1,870 2,160 
275 1,980 2,290 
300 2,090 2,420 
have extended to somewhat more than 600 kilometers below the level 
of isostatic adjustment, which, according to Bowie (8), is now con- 
sidered to be at a depth of about 96 kilometers, or 60 miles. Below the 
level of isostatic adjustment, the surfaces of constant pressure and 
constant temperature are perfect ellipsoids of revolution, while above 
this level these surfaces become more and more irregular as the sur- 
face of the earth is approached. One of the outstanding problems of 
the geologist is first to locate accurately the isogeothermal surfaces 
and second to relate them to heat sources past and present. 
Neglecting the radioactivity of the sedimentary rocks, Jeffreys (9) 
concludes that nearly all of the heat that escapes from the surface of. 
the earth is generated in a granitic layer which has a thickness of 
about 11 kilometers. Beneath this layer is an intermediate layer which 
is practically free from radium. It has a thickness of 22 kilometers. 
The discontinuities at 11 and 33 kilometers are supposed to be in 
agreement with seismological data. From 33 kilometers to 2,900 kilo- 
meters, another discontinuity, there appear to be no special changes in 
the properties of the rocks in so far as the transmission of earthquake 
waves are concerned. These results, unfortunately, are subject to 
correction on account of the difficulty of interpreting seismological 
data, but they are of interest to us in showing the possibility of cor- 
relating the temperatures and densities of the rocks in the outer shell 
of the earth. By taking into account a wide range of phenomena, in- 
cluding the evidence of seismology, chemistry, geothermics, cos- 
mogony, geology, and so on, Daly (10) places the discontinuities at 
30, 40, 60, 1,200, and 2,900 kilometers. 
ISOSTASY 
It has been found by geodetic observations that the crust of the 
earth extends to a depth of about 100 kilometers, or 60 miles, below 
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