
 Parrl.] CONDITION OF THE EARTH'S INTERIOR. 49 
same mass, heat being more easily conducted along than across the 
-“oyrain,”’ the bedding, and the other numerous divisional surfaces. 
_ Experiments have been made to determine these variations in a 
— number of rocks. Thus, the conductivity in a direction transverse 
_ to the divisional planes being taken as unity, the conductivity 
paralle] with these planes was found in a variety of magnesian schist 
to be 4028. In certain slates and schistose rocks from central 
France the ratio varied from 1: 2°56 to 1: 3952. Hence in such 
fissile rocks as slate and mica-schist, heat may travel four times 
more easily along the planes of cleavage or foliation than across them.* 
In reasoning upon the discrepancies in the rate of merease of 
subterranean temperatures, we must also bear in mind that certain 
kinds of rock are more liable than others to be charged with water, 
and that, in almost every boring or shaft, one or more horizons of 
such water-bearing rocks are met with. The effect of this inter- 
stitial water is to diminish thermal resistance. Dry red brick has 
_ its resistance lowered from 680 to 405 by being thoroughly soaked 
in water, its conductivity being thus increased 68 per cent. A piece 
of sandstone has its conductivity heightened to the extent of 8 per 
cent. by being wetted.’ 
Mr. Mallet has contended that the variations in the amount of 
increase in subterranean temperature are too great to permit us 10 
believe them to be due merely to differences in the transmission of the 
general internal heat, and that they point to local accessions of heat 
arising from transformation of the mechanical work of compression, 
which is due to the constant cooling and contraction of the globe.* 
But it may be replied that these variations are not greater than, 
from the known divergences in the conductivities of rocks, they 
might fairly be expected to be. 
Probable Condition of the Earth’s Interior.—YVarious 
theories have been propounded on this subject. There are only three 
which merit serious consideration. (1.) One of these supposes the 
planet to consist of a solid crust and a molten interior. (2.) The 
second holds that, with the exception of local vesicular spaces, the 
globe is solid and rigid to the centre. (8.) The third contends that 
while the mass of the globe is solid, there lies a liquid substratum 
beneath the crust. 
| 1. The arguments in favour of internal liquidity may be summed 
up as follows. (a.) The ascertained rise of temperature inwards 
from the surface is such that, at a very moderate depth, the ordinary 
melting point of even the most refractory substances would be 
reached. At 20 miles the temperature, if it increases progressively, 
as it does in the depths accessible to observation, must be about 
“© Report of Committee on Thermal Conductivities of Rock,” Brit. Assoc. Rep. 1875, 
p. 61. Jannettaz, Bull. Soc. Géol. France (April-June, 1874), ii. p. 264. This observer 
has carried out a series of detailed researches on the propagation of heat through rocks, 
Which will be found in Bull. Soc. Géol. France, tomes i.—yi. (8rd series). 
? Herschel and Lebour, Brit. Assoc. Rep. 1875, p. 58. 
3“ Voleanic Energy,” Phil. Trans. 1875. 
