IMPRISONED HEAT. 99 



explanation of the temperature of deep waters, and of the 

 molten condition of rocky matter erupted from volcanoes. But 

 we know that boiling and melting points, under the enormous 

 pressure experienced within the earth, are materially higher 

 than at the surface. There is much reason also, to argue, on 

 theoretical grounds, that the rate of increase of temperature 

 continually diminishes at any considerable depths. But, though 

 the depth may be quite uncertain at which a rock-melting 

 temperature would be reached, we have the demonstration 

 that such temperature exists at some depth. 



Movements of temperature beneath the earth's surface are 

 slow. Many instances are known of permanent ice preserved 

 in caverns. At Brandon, Vermont, permanent frozen gravel 

 exists at a depth of sixteen feet. In the Caucasus, masses of 

 ice lie buried permanently in the moraines, one of which is 

 1,500 feet distant from any glacier, and others are a mile be- 

 low the termination of the glacier. In Siberia and in northern 

 America, the earth remains permanently frozen at a depth of 

 two or three feet. At Yakutsk, in eastern Siberia, the earth 

 is frozen to a depth of 700 feet. As these and other occur- 

 rences of permanent ice are not attributable to any climatic 

 influences now existing, they must be the records and evidences 

 of more rigorous climates in the past. In other words, the 

 climate of the present is still contending with temperatures 

 whose effects are lingering in protected situations long after 

 the climates have become ameliorated. It has been demon- 

 strated that an ice-cap resting several thousand years over any 

 considerable portion of the surface, would so reduce the sub- 

 jacent temperature of the earth that for many centuries after 

 the disappearance of the ice, a decrease of temperature would 

 be discovered in penetrating downward. Even centuries later, 

 so much cold would still remain within the earth, that the rate 

 of increase of temperature would be less than if the ice-cap 

 had not existed; and after 3,600 years, that rate would be 

 only half the normal rate. 



Masses of lava are singularly poor conductors of heat. A 

 lava stream has been found still burning a century after its 



