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. LIMESTONES FORMED OF ORGANIC REMAINS