COOLING OF THE EARTH 17 



which give rise to earthquakes, with twice this rigidity, 

 as Professor Milne informs me. Astronomical considera- 

 tions lead to the conclusion that its effective rigidity has 

 not varied greatly for a long period of past time. 



Still, while fully recognising these facts, the geologist 

 knows we all know that the crust of the earth is not 

 altogether solid. The existence of volcanos by itself 

 suggests the contrary, and although the total amount of 

 fluid material which is brought from the interior to the 

 exterior of the earth by volcanic action may be, and 

 certainly is, small from data given by Professor Penck, 

 I estimate it is as equivalent to a layer of rock uniformly 

 distributed 2 mm. thick per century ; * yet we have every 

 reason to believe that volcanos are but the superficial 

 manifestation of far greater bodies of molten material 

 which lie concealed beneath the ground. Even the 

 wide areas of plutonic rock, which are sometimes ex- 

 posed to view over a country that has suffered long- 

 continued denudation, are merely the upper portion of 

 more extensive masses which lie remote from view. The 

 existence of molten material within the earth's crust 

 naturally awakens a suspicion that the process of cooling 

 has not been wholly by conduction, but also to some 

 slight extent by convection, and to a still greater extent 

 by the bodily migration of liquid lava from the deeper 

 layers of the crust towards the surface. 



The existence of local reservoirs of molten rock within 

 the crust is even still more important in another connec- 

 tion, that is, in relation with the supposed " average rate 

 of increase of temperature with descent below the 

 ground." It is doubtful whether we have yet discovered 

 a rate that in any useful sense can be spoken of as 

 " average." The widely divergent views of different 



* The heat thus brought to the surface would amount to one 

 seventeenth of that conveyed by conduction. 



3 



