98 The Interior of the Earth. [Sess. 



poles was caused by the revolution of the earth on its axis 

 when in a plastic condition, clue to its high temperature at the 

 time. We have no direct evidence to prove that it ever was 

 so, though mathematicians think that the fact of the earth 

 having this particular shape is strong presumptive evidence 

 of a former plastic state. I believe physicists and men of 

 science generally assume that the earth was once in a molten 

 condition throughout, and that its present state as regards 

 temperature is due to cooling by radiation during long ages. 

 The solid crust which has formed in the meantime, and which 

 we inhabit, effectually cuts off from our observation whatever 

 may be happening in the depths below. Still, certain scraps 

 of evidence in different forms are afforded us, which, if inter- 

 preted aright, may help us to divine in some degree the state 

 of affairs underground. 



I think we may conveniently divide the inquiry into three 

 parts, — 1st, What is the temperature ? 2d, What is the con- 

 dition ? 3d, What are the materials, in the interior of the 

 globe ? 



1st, as to temperature. From the existence of volcanoes 

 in every quarter of the globe pouring out volumes of molten 

 lava and hurling fragments of incandescent rock a great height, 

 while the clouds above reflect far and wide the light from the 

 glowing furnace of their open craters, we cannot doubt that 

 some part of the interior must be at a very high heat indeed, 

 sufficient at least to render refractory rocks perfectly fluid. 

 We are equally sure that these manifestations of high temper- 

 ature, so near the surface, are very local. Measurements of 

 the heat in deep mines and borings all point to the conclusion 

 that we must go far deeper than we have ever yet been, before 

 we meet with anything like the degree of heat commonly 

 exhibited at active volcanic centres. 



The influence of the changes of temperature brought about 

 by climate does not penetrate very far into the earth's crust, 

 and it has been estimated that at a depth of about 50 

 feet beneath any given place on the land a thermometer would 

 mark the same temperature all the year round. This point is 

 called the " limit of seasonal variation." Below this point, as 

 far as we have been able to pierce, it is found that the tem- 

 perature of the rocks always increases with the depth. This 



