Page:The Habitat of the Eurypterida.djvu/115

 lime shows under the microscope an exceedingly fine-grained lime mud, the grains being angular and of varying sizes, with rhombic crystals of dolomite scattered through the mass of calcite fragments. There are also many fine, black specks, probably of carbonaceous material. The most significant fact of the composition, however, is the presence of the silica and the alumina, which forms nearly one-third of the rock. Such a composition is entirely incompatible with the idea of chemical deposition, where we should expect practically pure carbonates.

(b and c) Organic origin. If the Bertie were an organic deposit its fine texture would permit of only two types of organisms active in its formation, namely, the protozoa or the algae. The lime content might be supplied by Foraminifera or by lime-secreting algæ, the silica by Radiolaria. The microslide of the Bertie shows no trace of any of these organisms. One other method of organic deposition is possible. The work of Drew, Sanford, and Vaughan has recently shown that in warm or tropical seas certain bacteria are active in precipitating calcareous muds from the sea water. That the Bertie waterlime could not have had such an origin is evident from its chemical composition given on page 106 above, in which the silica and alumina play too important a part, amounting to 28.98 per cent of the whole.

Since the chemical and microscopic study of the Bertie proves the impossibility of either a chemical or an organic origin, we must conclude that the rock is clastic.

(d) Clastic origin. A rock of clastic origin may have one of two sources: (1') it may be composed of material which was originally derived from the sea, that is, it may be thalassigenous, or (2') it may be derived from the erosion or breaking up of a pre-existing rock on the land, that is, it may be of terrigenous origin.

(1') Organic material broken up in the sea by organisms, or along the shore by waves, consists of shells, corals, and other hard parts of organisms mixed with varying amounts of sands and muds, organic and inorganic, the composition depending on the character of the rock supplying the detritus. Such clastic deposits are especially well developed around coral reefs where the purely biogenic rocks grade laterally in all directions into the clastic ones. That the Bertie waterlime could not have been a lime mud derived from the erosion of coral or other reefs and deposited in the surrounding quieter water or in the lagoons, as in the case of the similar, fine-grained lime mud