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titical incrustations of caverns, &c. may be referred to the samegeneral head. In many instances these recent calcareous depo-sits include land or freshwater shells, leaves, and other sub-stances which have become accidentally entangled in them.

But of all the changes resulting from causes in actual opera-*tion, those depending on the agency of volcanos are the mostremarkable. By this cause new mountains are still occasionallyelevated, and new islands formed ; thus the Monte Nuovo,elevated above 600 feet, rose in the space once occupied bythe Lucrine lake in the year 1538. See Hamilton’s letters onVolcanos, p. 127 ; and more receutly (1759) in Mexico , fromsimilar causes, a tract of ground of four square miles in extent,swelled to the height of 524 feet, and in the centre of a thou-sand burning cones, six large masses elevated themselves morethan 1200 feet; the loftiest summit, known as the Volcano ofJorullo, rising no less than 1695 feet. AY hat renders the casemore remarkable is, that the point where this extraordinaryconvulsion happened, is more than 42 leagues distant from everyother volcano. See Humboldt ’s New Spain, vol. 2. p. 165.The islands of the Greek Archipelago, especially those of Hieraand Santorini , have received accessions by the extension ofthose previously existing, and the formation of new islets in theadjoining sea by volcanic eruptions, in 726, 1427, 1573, and1650. New islands are recorded as having been thus formedamong the Azores in 1628, and very recently in 1811.

By the same cause extensive districts are covered with thickbeds of volcanic ashes; those resulting from successive erup-tions being regularly disposed, stratum super stratum, and over-flowed by long currents of lava, or sometimes by streams ofejected mud. Mackenzie’s and Henderson’s travels in Iceland ,Fortis, Spallanzani, Dolomieu, and Ferrara in Italy and Sicily ,and Humboldt’s in South America , should be referred to onthese heads.

Closely connected with volcanic phoenomena are those ofearthquakes. These often produce subversions of the strata,which represent, though on a smaller scale, and may perhapsaccount for, those grand dislocations and derangements whichwe generally observe in the beds composing mountain chains.See particularly the description of the effects produced by thegreat earthquake in Calabria .

The agency of volcanic powers appears to have been muchmore extensive at an earlier period, even under the existingorder of things, and subsequently to the last great convulsionof the Earth ’s surface, than they are at the present moment;for we observe craters now extinct of the most indisputablecharacter, which have covered with showers of scoria; many