and Leicestershire Dolomites. 249 



can be considered to yield any reliable evidence concerning dolomitiza- 

 tion in general. 



3. Inherent Structueal Evidences. 



(i) The degree of idiomorphism of dolomite crystals may afford 

 subsidiary evidence concerning the class to wliich the dolomite 

 may be assigned.^ In many cases where subsequent dolomitization 

 is amply proved by other evidences, it is found tbat the rhombohedra 

 are considerably more idiomorphic than those of most dolomites of 

 undoubted contemporaneous or primary origin. In tbe latter cases 

 there is a marked tendency of the crystals to interfere with one 

 another, the resulting structure being amoreorless granular mosaic. 



(j) The size of the rhombohedra is usually larger in subsequent 

 dolomites w^here the growth of crystals is less impeded than it is in 

 contemporaneous and primary dolomites. - 



(Ic) As in the case of idiomorphism and size, the degree of purity 

 of a dolomite is suggestive, but by no means conclusive.^ 



Crystals of a primarily precipitated dolomite should, in general, be 

 less contaminated with impurities than those of a dolomite of 

 secondary origin, though a contemporaneous dolomite may attain 

 a fair degree of purity. It is, perhaps, safer to place no reliance 

 upon the degree of purity. 



(/) The relation of iron oxides, particularly hsematite, to the 

 dolomite rhombohedra, affords one of the most reliable evidences 

 concerning subsequent dolomitization.* When haematite is included 

 either centrally or zonally in the rhombohedra the only possible 

 conclusion is that the haematite was introduced at the time when the 

 dolomitization took place. For instance, in a district where Trias 

 comes above Carboniferous dolomites having zonal inclusions of 

 haematite, the obvious inference is that the dolomitization was 

 subsequent and associated with waters percolating through the Trias 

 (see micro-photograph, PI. XI, Fig. 1). On the other hand, should 

 the haematite be only interstitial, the inference is that dolomitization 

 took place before the introduction of iron oxide. In a case where 

 Trias rests upon Carboniferous dolomites, the presence of only 

 interstitial haematite in the dolomite would indicate that the 

 dolomitization was certainly Pre-Triassic, perhaps contemporaneous. 

 Inferences similar to those made from the presence of included 

 hasraatite appear to be justified in cases where dolomitization is 

 intimately associated with other ores such as galena and zinc blende.^ 



(in) The relation of rhombohedra to chert in cherty dolomites 

 affords definite evidence with regard to the relative periods at which 

 the dolomite and chert were respectively formed.^ If rhombohedra 

 occur enclosed by chert, the inference is that the dolomite was 

 formed either before or simultaneously with the chert. On the 



^ See F. M. Van Tuyl, Iowa Gaol. Surv., vol. sxv, p. 390 et seq., 1916. 



- Swansea (Mem. Geol. Surv.), 1907, p. 16. 



^ F. M. Van Tuyl, Iowa Geol. Surv., vol. xxv, p. 319. 



* Sioansea (Mem. Geol. Surv.), 1907, pp. 15, 16. 



^ Schmidt, Trans. St. Louis Acad. Sci., vol. iii, No. 2, 1875, p. 246. 



^ H. H. Thomas, Ammanford (Mem. Geol. Surv.), 1907, p. 76. 



