PLANS FOR OBTAINING SUBTERRANEAN TEMPERATURES. 263 



a boring traversing a series of diverse rocks might be corrected 

 for the conductivities of the several rocks, but the determination of 

 subterranean conductivities is a matter of such difficulty that a trust- 

 worthy correction can not be applied, and the difficulty can be met 

 only by avoiding the necessity for correction. The first condition, 

 therefore, to be satisfied in the selection of a locality for a boring is that 

 the rock be of uniform character for the whole depth of the boring. 



Continuity of Rock. — The disturbing factor which impairs most 

 records of subterranean temperature is subterranean circulation of 

 water. There are few districts of sedimentar}' rock exempt from 

 subterranean circulation. Descending currents entering regions of 

 higher temperature receive heat from the rocks they traverse, and 

 this heat is carried to the surface by ascending currents. Thus con- 

 vection partly replaces conduction as a conveyor of heat, and the 

 conditions are rendered unfavorable for the development of the nor- 

 mal temperature gradient. As circulation is promoted by all cracks 

 and other partings of the rock, as well as by porosity, it is important 

 that districts where these occur be avoided. The second condition to 

 be satisfied in the selection of a site for a boring is that the rock be 

 continuous, or massive, and impervious. 



Topography of the Surface. — Every modification of the earth's sur- 

 face causes a modification of the subjacent isogeotherms, and if the 

 change is rapid it causes a temporary irregularity in the isogeotherms 

 near the surface. If the result of the topographic change is a plain, 

 the isogeotherms eventually become parallel planes with regular in- 

 tervals ; but if the result of the topographic change is a surface of 

 bold relief, the isogeotherms tend toward an adjusted distribution 

 which reflects the topographic irregularities. 



From these considerations arise two conditions to be taken into 

 account in the selection of a place for boring. It should be a plain 

 or surface of low relief, and the plain should be one which has not 

 received a heav^y deposit during the later geologic periods. 



Stability of Stir face Co7iditio7i. — The temperature of the surface of 

 the ground is ordinarily determined by the mean annual temperature 

 of the air. The temperature of the bed of the ocean is similarly de- 

 termined by the temperature of the water ; and the temperature 

 beneath a glacier is determined by the basal temperature of the ice, 

 which is approximately 0° C. These surface temperatures are the 

 initial or control temperatures to which the isogeotherms conform. If 

 they are changed, a readjustment of isogeotherms is at once instituted , 

 and during the period of readjustment the spacing of isogeotherms, or 



