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Show 380 RELATIVE AGE OF [Ch. XXVI. but in order to see them, we must travel to lands near the equator. In like manner, if the hypogene rocks can only originate at great depths in the regions of subterranean heat, and if it requires many geological epochs to raise them to the surface, they must be very ancient before they make their appearance in the superficial parts of the earth,s crust. They may still be forming in every century, and they may have been produced in equalquantitiesduring each successive geological periodof equal duration; but in order to see them in a nascent state, slowly consolidating from n state of fusion, or semi-fusion, we must descend into the 'fuelled entrails' of the earth, into the regions described by the poets, where for ages the land has --· ever burn'd With solid, as the lake with liquid fire. As the progress of decay and reproduction by aqueous agency is incessant on the surface of the continents, and in the bed of the ocean, while the hypogene rocks are generated below, or are rising gradually from the volcanic foci, thus there must ever be a remodelling of the earth's surface in the time intermediate between the origin of each set of plutonic and metamorphic rocks, and the protrusion of the same into the atmosphere or the ocean. Suppose the principal source of the Etnean lavas to lie at the depth of ten miles, we may easily conceive that before they can be uplifted to the day several distinct series of earthquakes must occur, and between each of these there might usually be one or more periods of tranquillity. The time required for so great a development of subterranean eleva tory movements might well be protracted until the deposition of a series of sedimentary rocks, equal in extent to all onr secondary and tertiary formations, had taken place. 1Ve conceive, therefore, that the relative age of the visible plutonic and metamorphic rocks, as compared to the unaltered sedimentary strata, must always be determined by the relations of two forces,-the power which uplifts the hypogene rocks, and that aqueous agency which degrades and renovates the earth's Ch. XXVI.] JlYPOG I~NE FORMATIONS. 38l surface; or, in other words, it must depend on the quantity of aqueous action which takes place between two periods, that when the heated and melted rocks are cooled and consolidated in the nether regions, and that when the same emerge to the day. Volume of hypogene rocks supposed to have been formed since the Eocene period.-If we were to indulge in speculations on the probable quantity of hypogene formations, both stratified and unstratified, which may have been formed beneath Europe and the European seas since the commencement of the Eocene period, we should conjecture, that the mass has equalled, if not exceeded in volume, the entire European continents. The grounds of this opinion will be understood by reference to what we have said of the causes which may have upheaved part of Sicily to a great height above the level of the sea since the beginning of the Newer Pliocene period*. If the theory which, in that instance, attributes the disturbance and upheaving of the superficial strata to the action of subterranean heat be deemed admissible, the same argument will apply with no less force to every other district, elevated or depressed, since the commencement of the tertiary period. But we have shown, in our remarks on the map of Europe, in the second volume, that the conversion of sea into land, since the Eocene period, embraces an area equal to the greater part of Europe, and even those tracts which had in part emerged before the Eocene era, such as the Alps, Apennines, and other mountain-chains, have risen to the additional altitude of from ] 000 to 4000 feet since that era. · We have also stated the probability of a great amount of subsidence and the conversion of considerable portions of European land into sea during the same period-changes which may also be supposed to arise from the influence of subterranean heat. Ft·om these premises we conclude, that the liquefaction and alteration of rocks by the operation of volcanic heat at sue- * See above, p. 107. |