1893-94-] The Interior of the Earth. 103 



This theory of a fluid zone in the earth's crust, if sound, is 

 of great importance to geologists, who would otherwise have 

 some difficulty in accounting for many geological facts con- 

 nected with earth movements if the globe were considered to 

 be rigid throughout. Assuming the existence of the fluid sub- 

 stratum, Mr Fisher, from physical considerations, assigns an 

 average thickness of 25 miles to the outer crust at the sea- 

 level, and that it is thicker under mountain- chains and thinner 

 under the oceans. 



The conclusions to be drawn from a survey of all the fore- 

 going observations regarding the state of the interior of the 

 earth may be stated provisionally as follows : 1st, That the 

 interior is very hot. 2d, That a portion of it is liquid, and 

 forms a continuous layer between the solid crust and the 

 central mass, whether that be solid or plastic. 3d, That the 

 solid crust is comparatively thin. 4th, That the liquid layer 

 is saturated with dissolved gases. 



With regard to the third point of our inquiry — namely, the 

 nature of the materials composing the interior of the globe — it 

 is well to realise at the outset what a very short distance we 

 have been able to pierce the crust by means of mines or bor- 

 ing : a mile is beyond the greatest depth so reached, and this is 

 less than the ^oo P ar ^ °^ ^ ne distance to the centre. For- 

 tunately, we have two means of observing the nature of sub- 

 stances from great depths — first, by means of matter ejected or 

 poured out by volcanoes ; and secondly, in the character of the 

 plutonic rocks, underlying or injected among stratified rocks, 

 and laid bare by detrition of the superincumbent strata. 



The most important evidence of the nature of materials from 

 the interior is furnished by astronomical data. It is found that 

 the average density of the matter forming the whole earth is 

 about 5h times that of an equal bulk of water. This is not 

 mere conjecture, but may be considered as proved beyond con- 

 troversy. Now the mean density of the substances composing 

 the crust, as far as our greatest range of observation extends, 

 varies from 2 \ to 3 times that of water. On these grounds, 

 we are forced to the conclusion that at very great depths 

 indeed the materials are of much greater density than at the 

 surface ; and it seems that that density must be at least twice 

 as great. It is difficult to say to what extent matter may 



