IGNEOUS AGENCIES. 145 



and vapors, especially steam. The power of these agents 

 is well known ; and gas and steam issue in immense quan- 

 tities during eruptions, especially of the explosive type. 

 On this point there is little difference of opinion. 



(#.) Heat. But the cause of the heat necessary to fuse 

 the rocks is one of the most difficult of all questions con- 

 nected with the physics of the earth. By most geologists 

 it is thought to be connected with the primal heat of the 

 earth, and the supposed universal melted condition of 

 the interior. This view assumes (a) that the earth was 

 once an incandescent, fused mass. This is almost cer- 

 tainly true ; (b) that in cooling it formed a crust, which 

 thickened by additions to its inner face, until it is now 

 about thirty miles thick ; (c) and that this limit between 

 the solid crust and melted interior is the place of the 

 focus of volcanoes. There are many difficulties in the 

 way of acceptance of this view, some of which are given 

 on page 132. 



All other theories regard the melted matter as local, 

 but, as to the cause of the fusion, there is yet great diver- 

 sity of view. Some attribute it to chemical action ; some 

 to mechanical crushing. It must be remembered in this 

 connection,, however, that in some cases, at least, the 

 amount of heat required is not more than 800 F., for in 

 some lavas the fusion is hydrothermal, and in all cases 

 the access of water seems necessary to supply the force. 



Secondary Volcanic Phenomena. 



There are many phenomena which linger after the true 

 eruptions have ceased. The chief of these are hot springs, 

 carbonated springs, lime-depositing springs, solfataras, 

 fumaroles, mud-volcanoes, and geysers. These all seem to 

 be the result of circulation of water through lavas which 

 still retain their heat, and are therefore properly called 

 secondary volcanic phenomena. The lavas, outpoured by 

 primary or true eruptions, remain hot in their interior for 



LK CONTE, GEOL. 10 



