PROPOSED INVESTIGATION OF SUBTERRANEAN 

 TEMPERATURES AND GRADIENTS 



By G. K. Gilbert 



Washington, D. C, November ig, igo2. 

 To the Trustees of the Carnegie Institution. 



Gentlemen : I beg to submit for the consideration of the Insti- 

 tution a proposed investigation of subterranean temperatures and 

 gradients by means of a deep boring in plutonic rock. 



Information as to temperature gradients in the earth's crust de- 

 pends, up to the present time, on observations made in wells bored 

 for economic purposes and in mines. Wells bored for economic pur- 

 poses penetrate the sedimentary rocks, because such rocks contain 

 the potable waters, brines, petroleums, gases, and salt deposits which 

 are sought. For various reasons these rocks should not be expected 

 to give the normal temperature gradient of the earth's crust. First, 

 they are heterogeneous, and as different layers differ in heat diffu- 

 sivity, the gradient is modified thereby ; second, the temperatures of 

 the sedimentary rocks are, as a rule, modified by the circulation of 

 water ; since heat is thus distributed by convection, the conditions 

 are not favorable for the determination of the facts of conduction ; 

 third, the sedimentary rocks are not representative of the crust as a 

 whole ; therefore inferences from their temperature gradients and 

 other heat phenomena can not be applied to the igneous or plutonic 

 rocks which make up the body of the crust. 



Temperature observations in mines are also subject to exceptional 

 conditions, since the ore deposits followed by deep mines are often 

 associated with comparatively recent diastrophic or volcanic disturb- 

 ances, and are usually routes of water circulation. 



The student of the problems of the inner earth needs information 

 as to the thermal conditions of a homogeneous rock mass repre- 

 sentative of the crust as a whole. To this end it is desirable that a 

 boring be made, as deep as possible, in a plutonic rock. A mass 

 should be selected which is of great age and which has not for many 

 geologic periods been subjected to diastrophic changes. A mass 

 having large superficial extent and presenting uniform characters 

 throughout its area could presumably be penetrated to a great depth 

 without encountering important changes in composition. 



The method of boring adopted should be one which yields a core, 

 so that physical investigations can be made of ihe rock penetrated 



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