D. Forbes — On Volcanos. 316 



the object being, to impress forcibly upon your minds the opinion 

 that the various changes which have taken place in the earth's 

 history, inhabitants, and surface configuration, have been mainly 

 effected by what may be termed external agencies, i.e., by forces 

 acting from without, slow but sure in their operations, yet but 

 comparatively so feeble in their energy as to demand a practi- 

 cally unlimited amount of time being placed at their disposal 

 to enable them to accomplish those great revolutions which we see 

 and know to have taken place in the geological history of our globe. 



The object of the present discourse, however, is precisely the 

 reverse ; for at the same time that I wish you, as I do myself, to 

 realise in your minds and fully appreciate the immense importance 

 of the study of these slow, or as they have been called " uniformi- 

 tarian," agencies, I would request your attention in quite an opposite 

 direction, in order to point out to you, that if we wish to arrive at 

 sound conclusions on these subjects, we must constantly bear 

 in mind the fact that, from the very oldest periods in the earth's 

 history down to the present day, we have the most clear and incontro- 

 vertible evidence to prove the existence of internal agencies also, 

 that is, of forces acting from within the earth itself, which, from their 

 often being at the same time both intensely energetic and instan- 

 taneous in their operations, have been termed cataclysmic, in contra- 

 distinction to the so-called uniformitarian, or external agencies, before 

 referred to. 



Volcanos, which we are about to consider, are but so many exist- 

 ing proofs of the activity of such internal forces at the present mo- 

 ment, and as a geologist I may be almost pardoned if I regret, that 

 we do not in our happy isles possess even a single example of an 

 active volcano. Had we one, it would have immensely aided my 

 efforts to-day ; for as the old saying, has it, " out of sight, out of 

 mind," I feel that the previous lecturers have in this respect had a 

 great advantage over me, since a mere appeal to your experience and 

 observation at home could not but greatly assist them when ex- 

 plaining to you the effects of rain and rivers, or of ice, or the sea, 

 upon the surface of the ground. I, on the contrary, cannot point out to 

 you even the smoke of one of those subterranean furnaces which form 

 the subject of my discourse. 



Although I imagine that but very few of those now present have 

 ever witnessed a volcano in eruption, I feel quite sure that all must 

 have read accounts of such phenomena. During the last few years, 

 indeed I may say months, these have been of more than usually 

 frequent occurrence, and the newspapers have from time to time 

 brought us particulars of considerable volcanic outbreaks, both sub- 

 marine as well as terrestrial, in various parts of the world, very 

 distinct from one another. 



As regards the geographical distribution of recent volcanos, a 

 glance at the geological map of the world will suffice to show that 

 they are in reality scattered all over its surface ; yet it may be added, 

 more rarely occurring at any great distance from the sea, although 

 exceptional instances are met with far inland in all the four quarters 



