316 D. Forbes — On Volcanos. 



of the globe. In the north we find the volcanos of Iceland, Jan 

 Mayen, Kamschatka, Alaska, and others ; whilst the Antarctic voyages 

 of Ross prove that the mountains of the nearest accessible land to 

 the South Pole are also active volcanos. 



At the Equator, all but innumerable volcanos are seen in the 

 islands of the Indian and Polynesian Archipelagos, as well as in the 

 Pacific and Atlantic Oceans, and on the mainland of South America. 

 Midway between the Equator and the Poles are situate the volcanos 

 of New Zealand, the Canaries, Cape Verde, Azores, and Sandwich 

 Islands, as also those of Arabia, Eastern Africa, Mexico, Central 

 America, and the volcanos of the whole range of the Andes down to 

 Tierra del Fuego. 



Nearer home, Vesuvius, Etna, Stromboli, Santorin, and numerous 

 others in the Mediterranean, if not so grand in their dimensions as 

 some of those previously referred to, still present on the large scale 

 all the various aspects of volcanic phenomena, both submarine as 

 well as terrestrial. 



If now, however, we take a broader view of volcanic phenomena, 

 — and in addition to the before-mentioned still existing proofs of the 

 general distribution of volcanic centres, as they have been termed, we 

 also take into consideration the occurrence of eruptive rocks of similar 

 origin, which are everywhere found disturbing and breaking through 

 the strata of even the oldest rock formations, — it will be seen, afleast 

 as far as the geology of the earth's surface is at present known to us, 

 that there is scarcely a single area of any magnitude — of either the 

 land or sea — which at some period or other has not been broken 

 through or disturbed by what may be termed volcanic forces, acting 

 from within the mass of the earth itself ; and it is impossible to come 

 to other than the conclusion that these agencies must have played a 

 most important part in determining the main features of the earth's 

 external configuration as well in our times as throughout all periods 

 of its history. 



Should the question be asked. What is a volcano ? — the simplest re- 

 ply would be, " A hole in the ground deep enough to reach such 

 portions of the interior of the earth as are in a molten condition." 



In ordinary language, however, the appellation of "volcano" is 

 usually restricted to those cone-shaped mountains, from the hollow 

 summits of which flames, smoke, and vapours are at times seen 

 to ascend, and which occasionally break out into more imposing 

 activity by vomiting forth showers of ashes and fragments of in- 

 candescent rock, or by pouring out torrents of molten stone, to deluge 

 and devastate the unfortunate country in the vicinity. 



The numerous interesting descriptions and figures of volcanic 

 mountains, like Vesuvius and others, which have appeared in popular 

 as well as scientific publications, have no doubt made you all quite 

 familiar with the external features of a volcano, both when at rest 

 and in action, so that a description from me would be superfluous ; 

 the internal structure and mode of formation of each mountain re- 

 quires, however, some consideration, more especially as these points 

 were for a long time a subject of discussion among geologists. 



