Ch. XXIX] 



LAVA, AND SCOKIJE. 



487 



like comparing the roots of a tree with, its leaves and branches, which, 

 although they belong to the same plant, differ in form, texture, color, 

 mode of growth, and position. The external cone, with its loose ashes 

 and porous 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 

 jEtherias, tantum radice in Tartara tendit," 



for its roots do literally reach downwards to Tartarus, or to the re- 

 gions 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 destruc- 

 tion of whatever may once have formed the 

 uppermost part of ancient submarine or sub- 

 aerial volcanoes, more especially as those 

 superficial parts are always of the lightest 

 and most perishable materials. The abrupt 

 manner in which dikes of trap usually ter- 

 minate at the surface (see fig. 639), and 

 the water- worn pebbles of trap in the allu- 

 vium which covers the dike, prove incon- 

 testably that whatever was uppermost in 



these formations 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 visible in active volcanoes. 



It will be seen in the following chapters, that in the earth's crust 

 there are volcanic tuffs of all ages, containing marine shells, which bear 

 witness to eruptions at many successive geological periods. These tuffs, 

 and the associated trappean rocks, must not be compared to lava 

 and scoriae 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, al- 

 most everywhere in regions of active volcanoes, 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 examination of the igneous rocks of Sicily, 

 especially those of the Val di Noto, has proved that all the more ordi- 

 nary varieties of European trap have been there produced under the 

 waters of the sea, at a modern period ; that is to say, since the Mediter- 

 ranean has been inhabited by a great proportion of the existing species of 

 testacea. 



^., 



Fig. 639. 





=» o ° a oZX 







-11 



i II! 1 



Strata intercepted by a trap dike, and 

 covered with alluvium. 



