Cb. XXIII.] 



POSITION OF VOLCANIC VENTS. 



577 



s 









positions. Whatever doubts the philosopher expresses as to 

 the nature of these causes, it is assumed, as incontrovertible, 

 that the points of eruption will hereafter vary, because they 

 have formerly done so ; a principle of reasoning which, as I 

 have endeavoured to show in former chapters, has been too 

 much set at nought by some of the earlier schools of geology, 

 which refused to conclude that great revolutions in the earth's 

 surface are now in progress, or that they will take place 

 hereafter, because they have often been repeated in former 



ages. 

 Division of the subject. — Volcanic action may be defined to 



be ' the influence exerted by the heated interior of the earth 

 on its external covering/ If we adopt this definition, with- 

 out connecting it, as Humboldt has done, with the theory of 

 secular refrigeration, or the cooling down of an original 

 heated and fluid nucleus, we may then class under a general 

 head all the subterranean phenomena, whether of volcanos, 

 or earthquakes, and those insensible movements of the land, 

 by which, as will afterwards appear, large districts may be 

 depressed or elevated, without convulsions. According to 

 this view, I shall consider first, the volcano ; secondly, the 

 earthquake ; thirdly, the rising or sinking of land in countries 

 where there are no volcanos or earthquakes ; fourthly, the 

 probable causes of the changes which result from subterranean 

 agency. 



It is a very general opinion that earthquakes and volcanos 

 have a common origin ; for both are confined to certain 

 regions, although the subterranean movements are as a rule 

 by no means most violent in the immediate proximity of vol- 

 canic vents, especially if the discharge of aeriform fluids 

 and melted rock is made constantly from the same crater. 

 But as there are particular regions, to which both the points 

 of eruption and the movements of great earthquakes are 

 confined, I shall begin by tracing out the geographical 

 boundaries of some of these, that the reader may be aware 

 of the magnificent scale on which the agency of subterranean 

 fire is now simultaneously developed. Over the whole of the 

 vast tracts alluded to, active volcanic vents are distributed at 

 intervals, and most commonly arranged in a linear direction. 



vol. i. 



p p 





