* The granite-cliffs. l. 523. On long exposure to air the granites or porphories of this country exhibit a ferrugenous crust, the iron being calcined by the air first becomes visible, and is then washed away from the external surface, which becomes white or grey, and thus in time seems to decompose. The marbles seem to decompose by loosing their carbonic acid, as the outside, which has been long exposed to the air, does not seem to effervesce so hastily with acids as the parts more recently broken. The immense quan∣tity of carbonic acid, which exists in the many provinces of lime-stone, if it was extri∣cated and decomposed would afford charcoal enough for fuel for ages, or for the pro∣duction of new vegetable or animal bodies. The volcanic slaggs on Mount Vesuvius are said by M. Ferber to be changed into clay by means of the sulphur-acid, and even pots made of clay and burnt or vitrified are said by him to be again reducible to ductile clay by the volcanic steams. Ferber's Travels through Italy, p. 166.
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