CALCAHEOUS OOLIl^ES 759 



as suggested by Linck.^^ The oolites have since changed to the more 

 stable mineral calcite. In doing so the majority have retained their 

 original radial fibrous structure; but many have started to recrystallize 

 about a single calcite crystal as a nucleus and have continued to enlarge 

 this crystal until it occupies the greater part, or, in some grains, the 

 whole of the space originally occupied by the oolite grain (see plate 26, 

 figures 5 and 6). 



The dolomitic oolites in the upper part of the Beekmantown beds-^ are 

 not associated with fossils, but they occur near the top of a limestone and 

 dolomite series nearly 5,000 feet thick, in which other oolitic beds and 

 fossiliferous layers are found at various horizons. The writer has already 

 suggested that this whole series can best be explained as an ancient "coral 

 reef" (if that term can be so applied), built up by calcareous algse.^^ 

 If this is true, then the decaying tissue of these algae. would generate 

 sodium carbonate for the precipitation of the oolites. At other horizons, 

 where fossils are more abundant, ammonium carbonate was, no doubt, 

 the precipitating agent. 



The changes which these oolites have undergone since their formation 

 have been very different from those which affected the calcareous oolites 

 from the lower horizon. In ordinary light, under the microscope, the 

 individual grains are indicated only by a faint line which marks the 

 outer margin of the oolite. No nuclei are present, and there is neither 

 fibrous nor concentric structure within the grains. Between crossed 

 nicols the interiors of the oolites are seen to be made up of irregular 

 allotriomorphic grains of dolomite, not differing in any way from the 

 grains which fill the interspaces among the oolites. In most cases there 

 is a more or less complete ring of little rhombic crystals of dolomite 

 lying close to the line which marks the margin of each oolite grain (see 

 figure 3, plate 27). If it were not for these rings it would be impossible 

 to recognize any trace of oolitic structure with the nicols crossed, and 

 these rings are not prominent. 



From a study of these dolomitic oolites alone it would be impossible 

 to say whether the dolomitization had taken place contemporaneously 

 with the formation of the rock or at a later date. But when the siliceous 

 nodules are considered in connection with the associated dolomitic ma- 

 terial, it at once becomes evident that dolomitization occurred after the 



^ See also Murray and Irvine : Proc. Roy. Soc. Edinburgh, vol. xvii, 1889, p. 89. 



J. Walther: Abh. sacbs. Ges. Wissensch., Bd. 14 u. 16, 1888, 1891. 



Clarke : Bull. 491, U. S. Geological Survey, 1911, p. 136. 



28Bellefonte dolomite of Ulrich. Bull. Geol. Soc. Am., vol. 22, 1911, p. 657. 



» Jour. Geol., vol. xxi, 1913, pp. 237-244. 



