Ch. IX.] 



SUBTERRANEAN LAVAS. 107 



the gradual elevation of a tract of land nearly as large as 

 Iceland. We say nearly, because the lava which cooled down 

 beneath the surface, and under considerable pressure, would 

 be more compact than the same when poured out in the open 

 air, or in a sea of moderate depth, or shot up into the atmo- 

 sphere by the explosive force of elastic vapours, and thus con- 

 verted into sand and scoriae. 



According to this theory, we must suppose the action of the 

 upheaving power to be intermittent, and, like ordinary volcanic 

 eruptions, to be reiterated again and again in the same region, 

 at unequal intervals of time and with unequal degrees of force. 



If we follow this train of induction, which appears so easy 

 and natural, to what important conclusions are we led ! The 

 reader will bear in mind that the tertiary strata have attained 

 in the central parts of Sicily, as at Castrogiovanni, for example, 

 an elevation of about three thousand feet above the level 

 of the sea, and a height of from fifty to two thousand feet in 

 different parts of the Val di Noto. In this country, therefore, 

 we must suppose a solid support of igneous rock to have been 

 successively introduced into part of the earth's crust immedi- 

 ately subjacent, equal in volume to the upraised tract, and this 

 generation of subterranean rock must have taken place during 

 the latter part of the newer Pliocene period. The dimensions 

 of the Etnean cone shrink into insignificance, in comparison to 

 the volume of this subterranean lava; and, however stagger- 

 ing the inference might at first appear, that the oldest founda- 

 tions of Etna were laid subsequently to the period when the 

 Mediterranean became inhabited by the living species of testacea 

 and zoophytes, yet we may be reconciled to such conclusions, 

 when we find incontestable proofs of still greater revolutions 

 beneath the surface within the same modern period. 



Probable structure of the recent subterranean rocks of fusion. 

 Let us now inquire what form these unerupted newer Plio- 

 cene lavas of Sicily have assumed ? For reasons already ex- 

 plained, we may infer that they cannot have been converted 



