NORMAL GEOTHERMAL GRADIENT III 
An entirely different approach to the solution of the problem 
was proposed by the late G. K. Gilbert of the United States Geological 
Survey. In the first and third Year Books of the Carnegie Institution 
of Washington (1902 and 1904) he proposed the drilling of a deep well 
in plutonic rocks. The importance of this suggestion has been again 
emphasized by the recent observations of R. H. Cleland (7) in north- 
ern Ontario, Canada, which show that the reciprocal of the mean grad- 
ient in undisturbed areas of crystalline rocks may equal or exceed 200 
feet per °F. (109.7 meters per °C.). 
Jeffreys (9) estimates that subsidence to a depth of ro kilometers 
during a geologic period of 130 million years causes a rise in tempera- 
ture of about 250° C. at the plane of contact of the sediments and 
the crystalline rocks. As the heat of compression is less than 1° C., 
the rise in temperature is due almost entirely to the flow of radio- 
active heat into the sediments. With these facts in mind, let us con- 
sider the changes in the gradient at the surface of a sedimentary area. 
First, depression of isogeotherms immediately beneath ocean floors 
causes an increase in the flow of heat beneath oceans as compared 
with undisturbed land areas; and likewise, the sinking of the base 
of the sedimentary column into rocks of high temperature tends to 
increase the gradient during subsidence. Second, inasmuch as the 
total height of material eroded from a mountain range during the 
process of base leveling the range is several times the original height 
of the range (10), it follows that large quantities of heat in these 
areas are brought nearer to the surface of the earth by mass displace- 
ment. The net result of these activities is that in areas of sediments 
which have for one or more times been subject to uplift and subsi- 
dence, the total quantity of heat remaining in the rocks down to the 
level of concentric isogeotherms is probably at a minimum: the ob- 
served gradients at the surface, however, because of erosion and mass 
displacement outward in the vertical, are at a maximum. Thus, the 
absorption of heat during subsidence, and the subsequent erosion 
and displacement of rocks toward the surface, may account, in part, for 
the relatively high gradients found in sedimentary areas. Even thin 
sediments which are the remnants of extensive erosion, as at El 
Dorado, Kansas, may retain some of the absorbed heat. In undis- 
turbed areas, however, the gradients are probably at a minimum and 
represent a normal gradient in the sense that such a gradient is the 
result of an undisturbed flow of heat that began immediately following 
the solidification of the crust. As suggested by J. S. De Lury (11), a 
normal gradient as thus defined may equal or exceed 200 feet per 
degree Fahrenheit. Observations at Grass Valley, California, Frank- 
771 
