Poulett-Scrope — On Mallet^ s Theory of Volcanic Energy. 33 



tlie continents or the axes of their mountain ranges. • If we suppose 

 these continents to be the result of the unequal subsidence of large 

 areas of the crust, some remaining at high, others sinking to lower 

 levels, and forming the oceanic basins, it is intelligible that along the 

 bounding lines separating the sunken from the elevated areas, rents 

 should have been formed^ deep enough in some places to give 

 issue to some of the heat and mineral matter still in a fused or 

 viscous state beneath — rents which every subsequent disturbance of 

 the adjacent areas would probably deepen or enlarge, occasioning a 

 more or less interrupted sequence of eruptions. But it is not easy 

 to understand why, on Mr. Mallet's theory of the source of volcanic 

 energy, its development should be confined to these circa-continental 

 bands. It would seem more likely that intense crushing of the 

 subsiding rocks and conseqiient production of heat would occur 

 uniformly at every point of the spherical envelope ; or, if anywhere 

 at a maximum, that this would be beneath the centres of elevated 

 areas, rather than on their outer limits ; on which supposition 

 the once fused plutonic masses which so often form the axial 

 portions of elevated ranges, and throw off on either side the 

 superficial strata, may owe their expansion and rise in part to the 

 heat proceeding from the extreme crushing force concentrated at 

 those points. Yet even as regards these eruptive rocks, the 

 porphyries, granites, serpentines, etc., the presumption would seem to 

 be that their swelling up was owing to the expansion occasioned by 

 an increased flow of heat /row beneath into the crystalline and semi- 

 elastic subterranean matter, causing it to fracture and force up the 

 overlying beds of rock, and sometimes even to overflow the surface. 

 With regard to the modes in which volcanic heat, however origin- 

 ated, operates in fusing or liquefying portions of subterranean rock, 

 forcing it in this state up more or less deep and narrow fissures, and 

 expelling it on the surface as lava, or steam-ejected scoria, etc., 

 Mr. Mallet's views are neither original, nor, indeed, consistently 

 reasoned out. He follows those geologists, Lyell, etc., who consider 

 eruption's to be occasioned by the influx of water from seas or lakes 

 above through fissures into foci of heated lava below. And he 

 rejects as wholly untenable the notion that water could have origin- 

 ally existed in molecular combination with the crystalline matter 

 of the rocks before they were melted into lava. Yet he admits 

 that there is no limit to the depth to which water may infiltrate by 

 capillarity into the subterranean rocky matter, and that portions of 

 this so permeated with water may have been fused and reconsoli- 

 dated again and again. So also he ridicules the idea, which 

 he ascribes to Hopkins, of the existence of subterranean lalces, 

 or local reservoirs of lava. Yet he himself supports that of sepa- 

 rate foci, or localized portions of melted rock at different points 



^ Sucli rents may be compared to those produced along tlie edges of a slieet of ice 

 on a pond or river, from which the water beneath has been lowered in level ; or still 

 more closely to those lateral rents formed in a sheet of lava as its interior subsides on 

 cooling, or escapes laterally, of which the Almanayia in Iceland is a typical example. 

 See "Volcanoes," ed. 1862, p. 77. 



DECADE II. — VOL. I. — NO. I. 3 



