198 G. Poulett Scrope — On the Cause of Volcanic Action. 



dition, but already permeated with water), and by elevatory stocks 

 produces the earthquake-rents, and, if any one of these communicate 

 with the outer air, a volcanic eruption — the lava rising up the fissure 

 to the surface, and the water in it flashing into steam wherever dimin- 

 ished pressure permits, especially in the upper part of the vent, from 

 which the explosions always proceed. Nor is this view of the 

 phenomena of eruption opposed to the general principles of the most 

 eminent geologists, but the contrary. For example, Sir 0. Lyell, in 

 several passages of his Principles (see Ed. 1869, p. 233) represents 

 the original motive power of both earthquakes and volcanos to be the 

 lateral shifting of internal heat from one part of the subterranean 

 matter to another; increase of heat occasioning expansion and elevatory 

 movements accompanied by jarring rents causing earthquakes. If, 

 then, we believe the internal heated matter already to contain water, 

 which we know from examination does exist in all granitic and 

 metamorphic rocks, the production of rents in this manner, giving 

 partial freedom to the expansibility of the water, will account at 

 once for earthquakes and volcanic eruptions, without supposing a 

 flood of water from above suddenly to find its way down to and to 

 penetrate every part of the interior of the heated rock in some un- 

 intelligible manner. Sir C. Lyell, indeed (Principles, Ed. 1869, p. 

 233), supposes the rents that have admitted this water may be sud- 

 denly closed above, and the water converted into steam below find 

 its way somehow under a mass of fluid lava, which it may drive up 

 some neighbouring volcanic vent. But he has himself shown in an 

 earlier passage (p. 221) that it is only in the upper part of the column 

 of lava in a volcanic vent that the " red-hot or white-hot water en- 

 tangled in it, under tremendous pressure," is enabled by the reduc- 

 tion of that pressure to flash into steam, and produce the explosive 

 phenomena of a volcanic eruption ; and he fails to show — and does 

 not even seem conscious of the necessity for explaining — Jioio " atmo- 

 spheric or sea-water suddenly descending from above" could become 

 '• so entangled" within and throughout every part of the lava. 



I am aware that the theory I am opposing is considered to derive 

 support from the very general occurrence of volcanic vents within 

 or near to seas or large bodies of superficial water. But this same geo- 

 graphical position would result from the generally admitted fact that the 

 continental tracts have been elevated above the sea-level by internal 

 expansions of deeply-seated matter which could not force its way 

 outwards ; where, in the words of Mr. Mallet, " uncompleted efforts 

 to establish a volcano" have occurred. Where the effervescent matter 

 beneath has been enabled to find vent, there no elevation of the sea- 

 bottom (to any great extent) will have taken place, but rather subsidence. 

 And hence the great lines of volcanic eruption on the globe's surface 

 are found within or in the immediate vicinity of areas of subsidence ; 

 in other words, of seas or great inland lakes. And further, if we 

 suppose, as may well be conceded, that the elevatory action by which 

 chains of mountains have been (probably through a succession of 

 shocks) raised, is accompanied by the formation of more or less 

 distant downward opening rents parallel to the axis of the elevated 



