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Show 208 TRAVERTIN OF TJVOLT. tion, which contributes to the rapid growth of its delta. A large portion of the most splendid edifices of ancient and modern Rome are built of travertin, derived from the quarries of Ponte Leucano, where there has evidently been a lake at a remote period, on the same plain as that already described. llut, as the consideration of these would carry us beyond the times of history, we shall conclude with one more example of the calcareous deposits of this neighbourhood,-those on the Anio. Travertin of Tivoli.-The waters of the Anio incrust the reeds which grow on its banks, and the foam of the cataract of Tivoli forms beautiful pendant stalactites; but, on the sides of the deep chasm into which the cascade throws itself, there is seen an extraordinary accumulation of horizontal beds of tufa and travcrtin, from four to five hundred feet in thickness. The following seems the most probable explanation of their formation in this singular position. The Anio flows through a deep, irregular fissure or gorge in the Apennine limestone, which may have originated from subterranean move· ments, like many others of which we shall speak when treating of earthquakes. In this deep narrow channel there existed many small lakes, three of which have been destroyed since the time of history, by the erosive action of the torrent, the last of them having remained down to the sixth century of our era. We may suppose a similar lake of great depth to have existed at some remote period at Tivoli, and that, into this, the waters, charged with carbonate of lime, feU from a height inferior to that of the present cascade. Having, in their passage through the upper lakes, parted with their sand, pebbles, and coarse sediment, they only introduced into this lower pool, drift-wood, leaves, and other buoyant substances. In seasons when the water was low, a deposit of ordinary tufa, or tra· vertin, formed along the bottom; but, at other times, when the torrent was swollen, the pool must have been greatly agitated, and every small particle of carbonate of lime which was precipitated, must have been whirled round again ~and again in various eddies, until it acquired many concentric coats, so as to resemble oolitic grains. If the violence of the motion be suf· ficient to cause the globule to be suspended for a sufficient length of time, it would grow to the size of a pea, or much 'rRA VERTIN OF TIVOLf • 209 large·r . Small fraOo 'mcnts of VCO'etab1e st b · · the s1des of the stream, and the~ wa ~ms emg mcrusted on deus of oval globules and th shed m, would form the nu- produced by the restin' O' ofof ers of irre gu 1a r s1 1 apes would be o raD"ments £ r t' h of the basin, where aftet· a 0 • • 0 a Ime on t e bottom travertm. on one sid' e th cqmrm]dOo' an . unequa1 t 1u .c k ness of . . ' ey wou agam b t . . Sometimes globules, projcctin above e se m motiOn. stratum, would attract by chg . 1 ffith~ general level of a , em1ca a mt tl . · the act of precipitation and tl . y, o 1er matter m I . ' 1us growmO' on 1l 'd . t 1e exceptiOn of the point of t . o a st es, w1th spheroids nearly perfect and m con ~ct, ~Igh~ at length form might increase above and b l any eet m dtameter. Masses might afterwards present tl:e ow, I so that a vertical section 'Tivoli, where the nucleus of p 1enofmenon so common at some 0 the concentric circles |
