PART I. 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 

 " ^Etherias, tantum radice in Tartara tendit," 



for 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 always 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 

 m 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 



Strata intersected by a trap dike, and visible in active VOlcattOS. . 



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- 

 ria3 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 may be answered, that subterranean movements have caused, 

 almost everywhere in regions of active volcanos, great changes in 

 the relative level of land and sea, in times comparatively modern, 

 so as to expose to view the effects of volcanic operations at the 

 bottom of the sea. 



Thus, for example, the recent examination of the igneous 

 rocks of Sicily, especially those of the Val di Noto, has proved 

 that all the more ordinary varieties of European trap have been 

 there produced under the waters of the sea, at a modern period ; 



