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."Pouzzolana. 4 T 3 it cannot readily pervade the caked argill thst invefts the ferruginous particles, yet in time even cold water may pervade it, and produce hardnefs; and hence lavas become harder when moiltened, as Mr. Dolomieu has obferved. Ponces, 437» If th' e niortar be long expofed to the attnofphere, fixed air, as well as pure- air, will uniteto the iron, ruft wftl he produced, and the mortar will not then harden, as Dr. Higgins has alfo noticed. Clay, over wldch lava has flowed, is freqqently c.onverted into pouzzolana. Ponces, 332. Büt volcanic fcorite never afford it; ibid. Seither becanfe thev are much caicined, or retain fulphur, or its acid. ■ _ * * « . * -V V* 3'd, TräfsJ orTerras-*. ' • ; I couple this with pouzzoiana, on account of their fimilarity to r each other, and not Wcanfe I loolc upon it as .coriftan#, and neccfTnrily,- a volcanic produäion. Ön the contrary,“ I beheve it to be generally the prodü&of pfeudo-volcauos, or external fircs. • . , It is foiind in many places, but principally near Andernach, in the vicinity of the Rhine, alfo nc2r. Frankfort, Cologne, Pleith, &c. and there called tuffstein. Its colour is grey brown or yellowilh. Its furface rough and porous. N Its luftre and tranfparency, .0. Itsfra&ure, couinionlycarthf, rarolylamellar, it contains fragments refembling pumice (thpugh. pot real pumice, Voight Fulda. 221); alfo frag- * 32 Mem. Stock. 51, ments
