_Travertine_ is a tufa laid down on twigs and other vegetation, where springs emerge laden with calcium carbonate. In a massive form, it builds tufa-basins, as in the Mammoth Hot Springs of the Yellowstone Park. Both here and at Karlsbad, it appears that vegetation of humble type, multiplying under warm conditions, materially assists the deposit by withdrawing carbon dioxide from the water. The unstable calcium bicarbonate is thus converted into the carbonate, which is thrown down as a quickly increasing crust. Among the limestone regions of the Dinaric Alps, calcareous tufas or travertines, laid down by ordinary streams, form massive beds that tend to choke the hollows of the hills. The basin of Jajce in Bosnia is thus partially filled up, and the town is built on materials brought in solution from the mountains. The modern waters are still adding to this deposit, and Fr. Katzer[4] has pointed out that the falls of the Pliva are prevented from cutting their way down to the level of the Vrbas ravine, into which they plunge, by the mass of tufa which they build up in their own course. Another type of limestone deposited from solution is of considerable interest in arid lands, or lands with only a seasonal rainfall. Where evaporation goes on steadily at the surface, while water is brought up by capillary action from below, calcium carbonate may form a cement to the soil, or to the crumbling rock near the surface, and a solid calc-tufa may arise by continued transference of matter in solution from lower levels. In the Cape of Good Hope such formations are conspicuous[5]. In a careful series of experiments, G. Linck[6] showed in 1903 that sea-water at 17° C. can only hold ·0191 per cent. of calcium carbonate in solution. Though this quantity is not realised in the open ocean, yet near shores rivers may bring down an excess. The Thames, though flowing for a long distance over a limestone area, contains only ·0116 per cent. of calcium carbonate; but springs traversing limestone often carry ·03 per cent., or ten times as much as that found in ordinary seas. Hence a precipitation of calcium carbonate from the bicarbonate state may take place not far from land. The mineral deposited is calcite in temperate climates and aragonite under warm tropical conditions. That such a precipitation actually occurs is proved by the massive grey limestones, containing modern shells, which have been recorded for our islands from the sea-floor off the Isle of Man and off the coast of Mayo. In the case of the Irish Channel, the excess of calcium carbonate may be supplied by springs rising through the glacial gravels, which contain abundant pebbles of limestone.