THERMAL CONDUCTIVITIES OF ROCKS 275 



gradients assembled from temperature measurements in boreholes show 

 the necessity of studying their significance. 



15. Applications of geothermal methods to the study of structures. — Detailed 

 application of diagrams showing the kinds of thermal fields to be expected 

 must be worked out in each district separately, having regard to local 

 structural and lithological conditions, and no general rules can be given for 

 their use. 



The author concludes that with regard to locating oil sands in a given 

 structure by means of temperature there are at present insufficient data on 

 the conductivity of oil-bearing beds in comparison with the conductivity 

 of the same beds when waterlogged or barren, and until these experimental 

 data come to hand geothermal data must be regarded as of use only in so 

 far as they are indicative of structure. 



Geothermic Gradients. 



By J. G. Finlay. 



South African Mining and Engineering Journal, vol. 41, part ii, no. 2051, 



i93i, PP- 509-510. 



The following brief description of Dr. Haldane's ' Calorometer ' suggested 

 for obtaining a standardised method of measuring rock temperature is given : 



The calorometer consists essentially of a brass tube about 1 in. in diameter, 

 containing a glass tube about \ in. in diameter. The space between the 

 glass and brass tubes is packed with dry felt, and the bottom and top of 

 the tube are secured as shown in a sketch. The glass tube is filled with 

 distilled water, a rubber plug is inserted, and the apparatus closed by means 

 of a brass cap. The calorometer is then placed in a bore hole and kept 

 there for say 18 to 24 hours, when it is quickly withdrawn, unscrewed, and 

 the temperature of the water taken. 



An average of over several hundred observations in Europe and America 

 gives what may be called a normal geothermic gradient of approximately 

 i° F. per 72 ft. descent. 



Great departures from this normal figure (Comstock Lode in Nevada, 

 i° F. for every 33 ft. descent ; Lake Superior copper mines, i° F. per 

 250 ft. descent) led to the conclusion that the geothermic gradient is in- 

 fluenced mainly by the following factors : 



(a) Lithological character of strata. — According to investigations by Dr. 

 Pirow it was found that the temperature gradient was steeper in the shales 

 than in the quartzites and that, generally speaking, the gradient was approxi- 

 mately proportional to the mean conductivity of the rocks. The following 

 table shows conductivity as given in the British Association Report : 



(b) Active chemical processes of decomposition. — Under such circumstances 

 the gradient tends to become steeper. 



(c) Relief of strata. — It has been demonstrated on several occasions that 

 when observations are taken on high ridges and mountains the gradient is 

 much flatter. 



