250 L. M. Parsons — Dolomitization 



other hand, should chert contain no included rhombohedra, the 

 formation of chert must have preceded that of dolomite, though this 

 does not necessarily prove subsequent dolomitization, since there is 

 no indication of any great period of time having elapsed between the 

 formation of chert and that of dolomite. 



4. Selective DoLoiiiTizATioif. 



The term "selective dolomitization" has been applied to certain 

 phenomena in which the formation of dolomite occurs more in certain 

 portions of rock, presumably the less coarsely crystalline and more 

 unstable parts, than in other more resistant portions. One result of 

 this differentiation is a rock of mottled appearance at times not unlike 

 a breccia. The relations between the dolomitic and non-dolomitic 

 portions of such rocks may afford evidence concerning the origin of 

 the dolomite. 



(w) Pseudo-brecciation, in which a limestone assumes a mottled 

 appearance on account of a patchy development of dolomite on 

 a small scale, can scarcely be considered to yield evidence analagous 

 to that of patchy dolomitization on a larger scale (/). That 

 a pseudo-breccia is unlikely to be due to the formation of primary 

 dolomite appears to be a legitimate inference, but the question as 

 to whether the dolomitization of any particular pseudo-breccia is 

 contemporaneous or subsequent must be decided by inherent evidence 

 other than that of mere mottling. Thus a rock of this kind in which 

 haematite is merely interstitial may be referable to contemporaneous 

 dolomitization.^ 



Any particular pseudo-breccia may supply evidence of its own 

 origin, but cannot form the basis for a definite generalization 

 concerning the mottling of dolomitic limestones. Each case must 

 be considered on its own merits. 



(o) A mottled appearance in which the dolomitic material is 

 worm-like or fucoid suggests dolomitization facilitated by the 

 presence of the remains of algse or other organisms." 



Of particular interest are cases of fossiliferous dolomitic limestones 

 in which selective dolomitization has differentiated between tbe 

 matrix and organic structures. It frequently happens that in 

 dolomitic rocks proved by general evidence to be of subsequent 

 origin, the matrix, whether calcite or aragonite originally, has been 

 largely converted into recrystallized calcite prior to dolomitization.^ 

 The recrystallized matrix, presumably on account of its relatively 

 coarse texture, is apparently more stable than the original calcareous 

 material of organic remains, consequently dolomite crystals liave 

 developed more in fossil structures than in the recrystallized matrix. 



{p) Hence it is inferred that differential dolomitization of this 

 kind may be an indication of subsequent altei-ation. That such 

 an inference may not always be legitimate is evident from the 

 occasional occurrence of dolomite crystals enclosed by recrystallized 



^ For instance, see Swansea (Mem. Geol. Surv.), 1907, pp. 14, 15. 

 ^ Peach & Home, North-West Highlands of Scotland (Mem. Geol. Surv.), 

 1907, p. 366. 



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



