1885.] Conductivity of Rocks, Sfc, 167 



Though this axis of the Alps is of late Tertiary date, the author points 

 out that it cannot be due to the protrusion of the granite, as the 

 Swiss geologists have shown that the granite was in its present rela- 

 tive position and solidified before the elevation of this last main 

 axis of the Alps, and he suggests that the higher temperature may 

 be a residue of the heat caused by the intense lateral pressure and 

 crushing of the rocks which accompanied that elevation, for in the 

 crushing of a rigid material such as rock, almost the entire mechanical 

 work reappears as heat. 



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

 Some of the apparent discrepancies in the thermometric gradients are 

 no doubt due to differences in the conductivity of the rocks. Apply- 

 ing the valuable determinations of Professors Herschel and Lebour to 

 the groups of strata characterising the several classes of observations, 

 the following results are obtained : — 



Mean Mean 

 conductivity. resistance. 

 k. " r. 



1. Carboniferous strata "00488 .... 275 



2. Crystalline and schistoze rocks. '00546 .... 184 



3. Triassic and cretaceous strata . . '00235 .... 465 



From this it would appear that the conductivity of the rocks associ- 

 ated with the mineral mines is twice as great as that of the artesian 

 wells class. But all the experiments, with the exception of three 

 or four, were made with blocks of dried rocks, and those shewed 

 a very remarkable difference ; thus, for example, dry New Red Sand- 

 stone gave 7jO'00250, whereas when wet it was increased to h 0*00600. 

 The author remarks that as all rocks below the level of the sea and 

 that of the river valleys are permanently saturated with water, dry 

 rocks are the exception, and wet rocks the rule in nature, consequently 

 the inequalities of conductivity must tend to disappear. The power of 

 conduction is also greater along the planes of cleavage or lamination 

 than across them, and therefore the dip of the strata must also exer- 

 cise some influence on the conductivity of different rocks and " massifs." 

 With respect to the foliated and schistose rocks, M. Jannettaz has 

 shown that the axes of the thermic curve along and across the planes 

 of foliation and cleavage, are in the following proportions : — 



Gneiss of St. Gothard 1 : 1'50 



Schists of Col Voza 1:1*80 



Cambrian Slates, Belgium 1 : 1*98 



This cause will locally affect the rock masses. 



Conclusion. — The author deduces from the three classes of observa- 

 tions a general mean thermic gradient of 48 feet per degree Fahr., but 



