LIMESTONES DEPOSITED FROM SOLUTION
Though calcium carbonate is far less soluble than calcium sulphate, large quantities are carried invisibly, owing to the presence of carbon dioxide, in river waters, and thus accumulate in inland seas that have no outlet except by evaporation. Here Calcareous Tufa may be deposited as a crust upon the shores and on the growing islets, as the water shrinks away, and before the more soluble gypsum and rock-salt can separate out. Hot springs of volcanic origin, like the Sprudel of Karlsbad in Bohemia, may deposit calcium carbonate as the water cools and is relieved from pressure. At Karlsbad, little grains of granite, or of the minerals of granite, serve as centres, and encrusting layers are formed round them, until pea-like bodies are produced. These become cemented together, giving rise to the well-known freshwater pisolitic limestone or roestone.
On the shores of the Great Salt Lake of Utah, calcareous tufa occurs also in the form of grains resembling little eggs. These are the oolitic grains that were first known as constituents of fossil limestones. The calcium carbonate of oolitic grains at Karlsbad, from the Great Salt Lake, and from the sea, is deposited in a form that gives the reaction of aragonite when boiled in cobalt nitrate. A. Lacroix, however, finds that the material at Karlsbad has a specific gravity lower even than that of calcite, and that its double refraction is also distinctly weaker. He has called this form of calcium carbonate "ktypeite."
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.
Ammonium carbonate, again, derived from the decay of organisms, or sodium carbonate, will precipitate calcium carbonate as aragonite from the calcium sulphate and chloride, but not from the calcium bicarbonate, of salt water. Films of aragonite are at present accumulating by this process on the floor of the Black Sea, and marine oolitic grains, also consisting of aragonite, are produced by the same reaction.
In the case of oolitic grains, deposition is no doubt helped by evaporation, since they seem to arise in shallow waters. The Oolitic Limestones that have proved so admirable as building stones, whether from the quarries of Caen or Portland, are cemented representatives of the loose deposits formed in modern tropical seas. De la Beche long ago compared their grains with those from West Indian coral-reefs. These small egg-like bodies develop round fragments of foraminiferal and other shells, round the ossicles of echinoderms, and round broken bits of coral. At first they have the general form of the nucleus; but, as they are rolled by the waves during their growth, they become more and more spheroidal as they enlarge. Boring algæ make tubular passages in them, and these have led to the view that algæ of thread-like form actually originate oolitic structure. Doelter, Linck, and others conclude, with much reason, that the mode of deposition is inorganic. When the grains are unusually large, they are often flattened and irregular, as in the marine Pisolites or Pea-grits.
For building purposes, the fine-grained oolites without large fossils are much sought after, since they can be trimmed equally in any desired direction.
Before leaving the question of the inorganic deposition of limestone, we may note that R. A. Daly[7] has suggested that the pre-Cambrian and early Cambrian limestones were entirely products of chemical precipitation. He believes that the continental areas were at first relatively small, and that the abundance of decaying soft-bodied organisms on the sea-floor led to a continuous precipitation of such calcium carbonate as was available. Hence the ocean was limeless, and it was only when continental land became more extended that a sufficient quantity of lime salts was brought in by rivers to counterbalance that thrown down by ammonium carbonate and sodium carbonate on the sea-floor. Daly urges that, on this account, the earlier organisms could not form calcareous shells or skeletons, and he also believes that pre-Cambrian and Cambrian limestones, even when unaltered, show no signs of having originated from fragmental organic remains. Linck's researches ([p. 17]) show that limestones thus precipitated must have originally consisted of aragonite.