On the Agency of Water in Volcanic Eruptions. 



165 



Granite 0'06 to 0*12 per cent. 



Devonian limestone 0'08 „ 



Basalt 0-33 



Slate 0'19 



Quartzite 0'66 „ 



Then, again, the conductivity, even when, dry, of the "ganister" 

 and other hard Coal-measure sandstones is found to be higher than that 

 of either the metamorphic or igneous rocks, averaging for four varieties 

 &=0'00737, r=136. 



It seems, therefore, probable that the unaltered sedimentary strata 

 are often, under their normal underground conditions, as good, if not 

 better, conductors of heat than the crystalline and igneous rocks 

 which constitute the greater proportion of the solid crust of the earth, 

 and, if so, the thermometric gradient would rather tend to become 

 more rapid in passing from the former into the latter than otherwise. 



But there is another cause which when the subject comes to be 

 more fully investigated will have to be considered — that is, the change 

 of conductivity produced by heat. Forbes found that the conduc- 

 tivity of iron varied with the temperature as under : — 



0° C 0-0133 



100 „ 0-0107 



200 „ 0-0082 



This shows a percentage decrement of the decrease of conductivity 

 of iron between 0° and 100® C. of 24*5, which agrees nearly, according 

 to Professor Tait, with the empirical law that the conductivity is 

 inversely as the temperature. Looking at other physical properties 

 which the metals and rocks have in common, it is not improbable that 

 they may have also this other property, although, no doubt, in a very 

 modified form. It is the more probable, inasmuch as the proportion 

 of iron present in the igneous rocks (commonly 10 to 15 per cent.) is 

 larger in those which are supposed to be the more deeply seated ; 

 while considering how largely the density of the mass of the earth 

 is in excess of the density of the crust, there is reason to believe 

 that the proportion of the metals increases with the depth. This 

 would produce an important effect on the thermometric gradient. 



A large number of the observations on temperatures at depths 

 have been conducted in carboniferous strata consisting largely of 

 sandstones of high conductivity; and others through chalk, clays, 

 and sands of which the conductivity becomes high when wet, — and 

 wet they must be at even small depths beneath the surface. On the 

 other hand, the crystalline and igneous rocks are so hard and compact 

 and absorb so little water, that their conductivity can be little 

 influenced by this cause. 



M 2 



