CHIEF DIVISIONS OF THE AQUEOUS ROCKS. 29 



able. It will not, therefore, be possible here to do more than glance 

 at some of the more salient peculiarities presented by oolitic lime- 

 stones. If a thin slice of any ordinary oolitic limestone be examined 

 under the microscope, it will be found to exhibit more or less 

 numerous rounded or oval grains, of variable size, embedded in a 

 matrix of crystalline calcite (fig. 11). Each oolitic grain, or spheroid, 

 ordinarily exhibits a more or less obvious structure out of concen- 

 trically superimposed layers, each layer being composed of minute 

 crystals of calcite arranged in a radiating manner, with their long 

 axes perpendicular to the surface. Very commonly there may be 

 detected in the centre of the grain a larger or smaller foreign body, 

 such as a grain of quartz or a fragment of some calcareous organism, 

 which has served as a nucleus round which the spheroid has been 

 built up. In other cases, no traces of a foreign nucleus can be re- 

 cognised. According to the view usually entertained, oolitic grains 

 of the type just described have been produced by " the original 

 deposition of calcite round nuclei gently drifted along by currents of 

 the ordinary temperature, which caught up more or less of the sur- 

 rounding mechanical impurities " (Sorby). According to this view, 

 therefore, the rock was primitively a loosely compacted aggregate of 

 oolitic grains, along with entire or fragmentary calcareous organisms, 

 and solidification was a secondary process, due to the percola- 

 tion through the mass of water charged with carbonate of lime 

 in solution, and the consequent precipitation of crystalline calcite 

 in all the vacant spaces between the grains. This view, doubtless, 

 affords an adequate explanation of the formation of the ordinary 

 oolitic limestones. There are, however, cases in which it would 

 rather seem that the formation of the oolitic grains has been due to 

 secondary crystallisation in an originally normal limestone. Thus, 

 in certain limestones some of the oolitic grains have no definite 

 boundaries, but consist simply of diffuse radiate crystallisations, 

 which may or may not have a central nucleus for their starting-point 

 (fig. 11). The structure just alluded to must, however, be carefully 

 distinguished from cases in which the oolitic grains have undergone 

 recrystallisation at some period posterior to their original formation. 

 In this latter case, the grains preserve their outlines, but the primitive 

 radiate and concentric structure is more or less completely destroyed, 

 and the spheroids consist simply of irregularly placed crystals of 

 comparatively large size. 



In all the fossiliferous formations, from the Ordovician onwards, 

 oolitic limestones are of common occurrence ; but they vary con- 

 siderably in their more minute characters. In one of the commonest 

 varieties of oolitic limestones the grains assume a greatly elongated 

 form, when the name of " spheroids " is hardly applicable to them. 

 Such elongated grains have been sometimes regarded as owing their 



