D. Forbes — On Volcanos. 323 



to the latest accounts, still continues to increase in size. Nume- 

 rous other examples might be cited, but I shall only mention the 

 island of Johanna Bogoslawa, in Alaska, which, although it only- 

 first showed itself above the water in May, 1796, had in 1806 

 increased so as to be an immense volcanic island, the summit of 

 which was then elevated to no less than 3,000 feet above the level 

 of the sea. 



The volcanic products thus forced out under the sea present, as 

 might be expected, a very different aspect from that of the ashes, 

 scoriae, and lava from terrestrial volcanos ; the molten lava, coming 

 in contact with the water, is at once broken up into fragments, coarser 

 or fi.ner in proportion to the greater or less cooling power of the 

 water in immediate contact with them, and often in great part in- 

 stantly converted into fine mud, of a greyish colour when formed from 

 trachytic lava, but more commonly of a chocolate or other dark tint, 

 and much denser when produced from the more prevalent pyroxenic 

 lava. Beds of this character, spread out by the action of the sea, 

 often inclosing shells, fish, and other organic remains, become in 

 time consolidated and upheaved, and as they often present an ap- 

 pearance much resembling ordinary volcanic rocks, they have fre- 

 quently puzzled geologists, who at first found a difficulty in explain- 

 ing the presence of such fossils in rocks apparently of igneous origin. 



As the limits of this discourse will not permit of my going further 

 into the consideration of these phenomena, or of even more than touch- 

 ing upon the chemistry or mineralogy of volcanos, as I should have 

 wished to do, I must now confine myself to some few remarks upon 

 the origin of volcanos, premising, however, since this subject is still 

 involved in great obscurity, that what I have to say on this head 

 must be received more as an expression of my own opinions than as 

 any yet received theory of volcanic action. 



Many writers on this subject hold to the belief that volcanos are 

 mere local phenomena, each one springing from its own compara-. 

 tively small reservoir of molten matter, supposed to have originated 

 from the softening or fusion of rocks pre-existing on the spot at 

 some depth below the surface. 



To me, however, this hypothesis appears altogether untenable, 

 when it is remembered, amongst other objections, which I have else- 

 where considered, that volcanic rocks are encountered in all parts of 

 our globe, often continuous or nearly so, over immense areas ; and 

 also that all these rocks, without reference to the part of the world 

 in which they occur, are invariably alike in character to one another. 



Volcanic rocks may be classified under two heads, viz., the dark 

 coloured, more dense; and the less heavy, light-coloured lavas — termed 

 respectively the basic or pyroxenic, and the acid or trachytic lavas. 

 Both these varieties may proceed from the same volcanic vent in 

 succession ; for instance, in Vesuvius, where the mineral matter which 

 buried Pompeii is trachytic, but the later lavas are generally pyrox- 

 enic in character ; this also was the case in the recent eruption of 

 ^Santorin, as reported upon by the Austrian Scientific Commission. 



The examinatiou of volcanic products, no matter how distant the 



