On Underground Temperatures. 



47 



Consequently granite and gneiss, with their large proportion of free 

 quartz, would be more affected than the other rocks. 



When, therefore, we consider at how late a geological period some 

 of the great mountain chains have been uplifted, it is not impossible, 

 looking at the magnitude of the massifs, that some residual portion of 

 the heat producedby compression, faulting, and crushing, may still exist 

 in such modern chains as the Alps and the Himalayas, or in Continental 

 areas of recent elevation when that elevation has been accompanied 

 by compression and faulting. This is a consideration which, although 

 exceptional, should not be overlooked in the general question of 

 underground temperatures, especially in mining districts, where we 

 have to deal with disturbed areas, with their faults, dykes, and 

 mineral veins ; at the same time, there can be little doubt that the 

 disturbances in most of these areas are of such high antiquity that 

 there is in most instances small probability of the rock showing 

 remaining traces of thermal effects due to these causes. Neverthe- 

 less, when the area has been affected by late disturbances, it is possible 

 for the thermic normal to be influenced by such a cause independently 

 of the action of any volcanic or igneous rocks.* 



Conductivity of the Rocks. Effects of Saturation" and Imbibition. 



Although it is evidently possible to account for many of the appa- 

 rent discrepancies in the thermometric gradients by the causes dis- 

 cussed in the foregoing pages, yet it is equally evident that there are 

 irregularities — not only between the rocks in the three groups of 

 observations, but also common to individual instances in each separate 

 group — which these causes do not adequately explain. As the rocks 

 in each group are of very different lithological characters, and as 

 there are also occasional lesser differences of characters in the 

 members of each separate group, the common disturbing cause may 

 in some measure depend upon those differences of structure and 

 composition which variably affect the conductivity of the rocks. 



The researches which bear most directly on this inquiry are those of 

 Professors Herschel and Lebourf in this country, and M. JannettazJ in 

 France, the former relating more especially to the differences de- 

 pendent upon the lithological structure of the rocks, and the latter to 

 those dependent on the component minerals. Tabulating the results of 

 Professor Herschel and Lebour's experiments in accordance with their 

 geological relations, as grouped in Tables II, III, and IV, we obtain 

 the following mean conductivities for the several groups of rocks : — 



# For a further discussion of this subject see a short paper by the author on 

 " Regional Metamorphism," in " Proc. Roy. Soc," vol. xl (1885), p. 425. 



f The results of their investigation are recorded in the reports of the British 

 Association for 1874-1882. 



X " Bull. Soc. Geol. de France," 3rd Ser., vol. hi, et seq. 



