100 WALKS AND TALKS. 



eruption. Some years ago a bed of ice was discovered on the 

 slope of JEtna, buried beneath a stream of lava ; and from this 

 the city of Catania has since obtained supplies of ice. On 

 Tierra del Fuego ice and lava are found interstratified for a 

 great depth each winter's snow being covered by a new lava- 

 sheet. In 1860 the crater of the mountain Kutlagaya, in Ice- 

 land, hurled out simultaneously into the air lumps of lava 

 and of ice, all intermingled together. 



These are interesting facts, but I propose for them no other 

 use at present than to show some possible reasons why the rate 

 of increase is unequal at different localities or different depths. 

 We know that some regions have been overlaid by sheets of 

 snow and ice. We have also discovered reasons for believing 

 that our northern States, as far as the bowlders are distributed, 

 were covered by continental glaciers during a geological 

 period. If this was so, it may be that their cooling influence 

 is still left within the earth ; and if it is, the rate of increase 

 of temperature as observed is lower than it would be under 

 normal conditions. A more rapid rate of increase implies a 

 thinner crust of solid matter. But, while these considerations 

 must not be forgotten, it must be confessed that most of the 

 questions concerning internal heat are still imperfectly un- 

 derstood. 



Though we are certain intense internal heat exists, we 

 neither know at what depth it exists, at what ratio it increases, 

 nor what is its cause or source. Nor do we know whether the 

 deep interior is in a solid or a liquid state. 



As to the cause of the heat, two principal theories are held. 

 The first conceives the internal heat to be the residual heat of 

 a cooling and once molten globe. (Talks, XXXVII and 

 XXXVIII). The earth is evidently cooling. The records of 

 past volcanic action prove that heat has escaped in enormous 

 quantities from the interior. Thousands of cubic miles of 

 molten lava now solidified over the surface, imply some reduc- 

 tion of the earth's temperature, and the problem is one which 

 might be solved. The traces of former intense action at the 

 surface are retained also in enormous rock formations which 



