t ( 53 r ) TWELFTH SPECIES. Calc Tuff. Kalk Tuff.—Werner. External CharaElers. t Colour yellow! fh grey, which fometimes approach es to fmoke grey. Seldom maffive, generally perforated, or marked With imprefllons of reeds, grafs or mofs alfo amor phous, ramofe, and corroded. Internally, luftre dull, feldom faintly glimmer ing. Fra&ure is intermediate between fine grained un- ev en and earthy; it ieldotn inclines a littie to fibrous ^nd foliated. Fragments indeterminately angular and blunt edg ed. Cpaque, and fometimes tranflucent on the edges. Soft, paffing into very loft. , i Somewhat mild. Fafily frangible, £‘ight, approaching to fwimming. * i k Geognostic