52 Prof. J. Prestwich. 



those planes. The lamination in micaceous sandstones produces a 

 similar result : — 



Conductivity. Eesistanee. 

 Slates, along the planes of cleavage. . 0"00561 184 

 „ across „ „ 0'00395 253 

 Micaceous flagstone, along the laminae 0'00690 145 

 across „ .. 0'00492 203 



M. Jannettaz* has extended the inquiry to a great number of other 

 rocks, and he shows that the variation in conductivity in many rocks 

 is largely dependent upon the preseuce of mica. He found that in a 

 crystal of mica, heat was conducted about two and a-half times more 

 rapidly along the planes of cleavage than perpendicular to it. In 

 augite these axes of the thermic curve are in the proportion of about 

 two to one. 



Fig. 6. — Mica. Fig. 7. — Augite (var. diopside). 



a, b, c, d, the thermic curve ; a, b, the major axis ; c, d, the minor axis. 



M. Jannettaz obtained results of a similar character, varying 

 according to certain physical conditions, in a number of other 

 minerals and in many rocks. The ratio of the minor to the major 

 axis in the following rocks he found to be as follows : — 



Gneiss of St. Gothard 1 : 150 



„ from near Chamouni 1 : 123 



,, passing into mica schist 1 : 163 



Schists (triassic), St. Gervais 1 : 1*50 



„ (carboniferous), Col Voza 1 : 1'80 



Argillaceous schists 1 : 1*25 



Cambrian Slate, Deville (Belgium) 1 : 1'86 



Fissile micaceous limestone 1 : 1*31 



Black and white limestone, Bonneville 1 : 1'06 



* " Bull. Soc. Geol. de France," 3rd Ser., vol. ii, p. 265 j vol. iii, p. 499, et seq. 



