Elements of geology / by Charles Lyell.

  • Charles Lyell
Date:
1838
Catalogue details

Licence: Public Domain Mark

Credit: Elements of geology / by Charles Lyell. Source: Wellcome Collection.

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    of pores or cellular cavities, which the entangled gases give rise to in ordinary lava. From these and other peculiarities it has been inferred, that the granites have been formed at great depths in the earth, and have cooled and crystallized slowly under enormous pressure where the contained gases could not expand. The volcanic rocks, on the contrary, although they also have risen up from below, have cooled from a melted state more rapidly upon or near the surface. From this hypothesis of the great depth at which the granites origi- nated, has been derived the name of 66 Plutonic rocks,” which they have received to distinguish them from the volcanic. The beginner will easily conceive that the influence of subterranean heat may extend downwards from the crater of every active volcano to a great depth below, perhaps several miles or leagues (see Frontispiece), and the effects which are produced deep in the bowels of the earth may, or rather must be distinct; so that volcanic and plutonic rocks, each different in texture, and sometimes even in composition, may originate simultaneously, the one at the surface, the other far beneath it. Although granite has often pierced through other strata, it has rarely, if ever, been observed to rest upon them as if it had overflowed. But as this is continually the case with the volcanic rocks, they have been styled from this pecu-
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