. Elements of geology. Geology. PART L CHAPTER VIII. 115 Relation of Trap, Lava, and Scorise. lava, may be likened to the light foliage and branches, and the rocks concealed far below, to the roots. But it is not enough to say of the volcano, " quantum vertice in auras " ^therias, tantum radice in Tartara tendit," fof its roots do literally reach downwards to Tartarus, or to the regions of subterranean fire; and what is concealed far below, is probably always more important in volume and extent than what is visible above ground. We have already stated how frequently dense masses
. Elements of geology. Geology. PART L CHAPTER VIII. 115 Relation of Trap, Lava, and Scorise. lava, may be likened to the light foliage and branches, and the rocks concealed far below, to the roots. But it is not enough to say of the volcano, " quantum vertice in auras " ^therias, tantum radice in Tartara tendit," fof its roots do literally reach downwards to Tartarus, or to the regions of subterranean fire; and what is concealed far below, is probably always more important in volume and extent than what is visible above ground. We have already stated how frequently dense masses of strata have been removed by denudation from wide areas (see Chap. VI.) ; and this fact prepares us to expect a similar destruction of whatever may once have formed the uppermost part of ancient submarine or subaerial volcanos, more especially as those super- ficial parts are alv/ays of the lightest and most perishable mate- rials. The abrupt manner in which dikes of trap usually termi- nate at the surface (see Fig. 104.), and the water-worn pebbles of trap in the alluvium which covers the dike, prove incontestably that what- ever was uppermost in these forma- tions has been swept away. It is easy, therefore to conceive that what is gone in regions of trap may have corresponded to what is now ^ intersected by a trap dike, and visiblc in activo VOlcanOS. covered with, alluvium. It will be shown in the second part of this volume, that in the earth's crust there are volcanic tuffs of all ages, containing marine shells, which bear witness to erup- tions at many successive geological periods. These tuffs, and the associated trappean rocks, must not be compared to lava and sco- rise which had cooled in the open air. Their counterparts must be sought in the products of modern submarine volcanic eruptions. If it be objected that we have no opportunity of studying these last, it maybe answered, that subterranean movements have caused, almost everywhere in regions of active volc
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