Mechanically-formed Limestones from Junagarh (Kathiawar), and other Localities

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Abstract
I. The Original Structures of Mechanically-formed Calcareous Rocks—Subaqueous, Littoral, and Æolian We are apt to think of the calcareous rocks as being laid down differently from other sedimentary strata. It is recognized at once that the deposition of the latter involves the mechanical forces of rivers, currents, waves, or winds ; but the former―when they are not the result of chemical precipitation―are often spoken of as if they represented merely the undisturbed accumulation of calcareous material, which either remains where the organism, of which it formed part, ceased to live, or sinks by its own weight at once to the sea-bottom. This distinction is, however, as every one would admit, only very partially valid. All the structures characteristic of the sedimentation of arcnaceous beds are exhibited, on occasion, by calcareous rocks ; though when these are formed in the sea, the phenomena are restricted and modified by the fact that carbonate of lime is soluble to a very large extent in water containing carbonic anhydride.¹ As a general rule, the water of rivers, lakes, and seas is not completely saturated with carbonate of lime, and therefore acts as a solvent of that substance. A small calcareous particle—whether it is derived from the erosion of a limestone; or is the result of the comminution of shells, corals, and other organic remains; or represents the test of a minute organism—will not remain long suspended in water which is still capable of taking up carbonate of lime, before it disappears into solution. Fine-grained

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