﻿GENERAL CONSIDERATIONS. 



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-specimens ; and very often sections even of the most promising limestones do not show 

 a single rhizopod test when submitted to microscopic examination. 



So far from owing its origin, like the true Chalk, chiefly to Foraminifera ; or indeed, 

 to go further, so far from being a deposit formed directly and exclusively by the agency 

 of animals secreting carbonate of lime, there are considerable areas of Carboniferous 

 Limestone in which the sea appears to have deposited its excess of mineral constituents 

 in accordance with chemical and physical laws, without the intervention to any great 

 extent of animal life. This has been brought about by a process of precipitation and the 

 subsequent coalescence of the impalpable particles of amorphous precipitate into minute 

 spheroids, the result being a concretional or oolitic limestone (often fossiliferous at the 

 same time), such as may be met with in formations of Devonian and Silurian as well as of 

 Carboniferous age. The constituent spheroids of such rocks have generally a radiate 

 structure, and in sections show one or more concentric rings ; the centre is often occupied 

 by a foreign body, such as a minute crystal, the fragment of a coral, or even a foraminifer, 

 though more commonly there is no observable nucleus. The section represented in Plate 

 XII, fig. 3, though showing a number of spheroidal concretions, is scarcely characteristic; 

 it would require a much larger space and a lower magnifying power to give quite a 

 correct idea of the general structure. The oolitic grains are normally nearly spherical, 

 but they also assume ovoid, elongate, or quite irregular forms, such shapes resulting either 

 from the partial coalescence of two or more spheroids, the distinct origin and structure of 

 which are easily traced up to a certain stage in the process of coalescence, or else from the 

 irregular outline of the foreign body upon which the precipitated carbonate of lime has 

 begun to arrange itself, the accumulation not having gone on long enough to produce a com- 

 plete sphere. There need be no difficulty in the acceptance of a physical explanation of 

 this sort, even by those who hold most firmly the theory that all limestones have primarily 

 an organic origin. It has repeatedly been urged that, to account for the azoic condition 

 of the deep-sea bed, in areas where evidence of animal life might have been expected, it 

 was necessary to remember the solvent power of water charged with carbonic acid ; — 

 that, especially under pressure, water so charged must dissolve the calcareous skeletons of 

 organisms subjected to its action. Of this fact there can be no doubt : what does not 

 appear to have been sufficiently taken account of is the converse, viz. that the solution 

 so formed is a very unstable one, and that, on the diminution of pressure, the elevation of 

 temperature, or other alteration of physical conditions, the carbonate of lime, so taken up, 

 is as rapidly precipitated, the form in which it presents itself on precipitation being 

 precisely the one most favorable to the process of spherical coalescence. 1 



1 It is, perhaps, needless to refer particularly to Mr. Rainey's elaborate researches on spherical 

 coalescence, as his papers are already well known, and they relate chiefly to the process as carried on in the 

 animal economy. The manufacture of carbonate of magnesia on the large scale from magnesian limestone 

 offers an excellent illustration of the solution of earthy carbouates in water charged with carbonic acid, their 

 precipitation by increase in temperature, and the subsequent coalescence of the precipitated particles. 



