On Underground Temperatures. 



51 



The following is the proportion of water held in rocks by complete 

 saturation : — * 





Complete 







OciLUl ctLlUll. 







0'06 in 



X\J\J J /til lO« 



ti "n fyvQinon 



0*12 



55 



kq do I j- An T7DT»n»n 





55 



Si nT»iQn olofocj A nrpafD 



0-1 Q 



55 





0'08 



55 





2-85 



55 



Coal-measures sandstone . . 



. 14-30 



55 





13-43 



55 





. 23-98 



55 



Calcareous freestone, Paris 



. 16-25 



55 



Chalk 



. 24-10 



55 



In the hard granites, sandstones, and limestones, the water of im- 

 bibition differs but little in proportion from that of saturation. The 

 difference is considerable in the softer rocks. The following are some 

 of the few experiments that have been made on this point. 



Quarry- 

 water. 



Gneiss, slightly decomposed 3'00 



Plastic clay 19*56 



Chalk 19-30 



It is clear then that the conductivity of the underground rocks 

 must, except in some very hilly districts, be taken solely as that of wet 

 rocks. In the harder and more compact rocks there will be little 

 difference, but in the softer and more porous rocks the difference 

 arising from this cause must be very considerable. 



The conductivity even of coal will be increased, although the 

 quantity of water that coal imbibes is very small. But unwrought 

 coal also contains a large proportion of gas — and gas in a state of 

 extreme condensation, or possibly in a state near liquidity, and this 

 also may have an effect upon its conductivity. 



Foliation and Cleavage. — The other condition, to which we have 

 already alluded, is that produced by foliation and cleavage, and by 

 the angle at which the strata lie. Messrs. Herschel and Lebour 

 showed, for example, that the conductivity of slates varies accordingly 

 as it is taken across or along the planes of cleavage — that while the 

 conductivity along the planes of cleavage is equal to that of the 

 crystalline rocks, it is no greater than that of soft sandstones across 



* These are on the authority of the late M. Delesse, "Bull. Soc. Greol. de 

 France," 2nd Ser., vol. xix, p. 64. 



E 2 



