324 REACTION OF THE INTERIOR OF THE EARTH 



points at which 3 within historic periods, the fluid in- 

 terior of the earth has remained in active intercourse 

 with the atmosphere. Such activity manifests itself, 

 most often simultaneously, by eruptions from volcanic 

 mountains, by increasing heat and inflammability of ther- 

 mal and naphtha springs, and in the increased extent of 

 areas of commotion: phsenomena which are all in intimate 

 interconnection and mutual dependence. ( 462 ) Leopold 

 von Buch has here again the great merit of having, in 

 writings appended to his physical description of the 

 Canary Islands, undertaken for the first time to em- 

 brace all the volcanic systems of the globe in a cosmical 

 view, while employing the well-grounded distinction of 

 central and linear systems of volcanoes. ' My own later, 

 and therefore more complete, enumeration, undertaken 

 on principles indicated above (pp. 243 and 265.) ex- 

 cluding, therefore, unopened domes, and mere cones of 

 eruption, gives, as a probable limitary mere minimum 

 number (nombre limite inferieur), a result which differs 

 considerably from any previously stated. 



The question has been repeatedly raised, whether in 

 the parts of the earth's surface in which volcanoes are 

 most numerous, and the reaction of the interior upon 

 the solid crust of the earth most apparently active, the 

 molten interior may not actually be nearer to the sur- 

 face ? Whatever be the mode adopted for determining 

 the greatest mean thickness of the solid crust, whether 

 the purely mathematical one supplied by theoretical 

 astronomy ( 463 ), or the more simple one which is based 

 on the law of increase of heat with increasing depth, and 

 the melting point of rocks ( 364 ) ; still the problem pre- 

 sents a great number of, at present, indeterminate quan- 



