and Leicestershire Doloinites. 251 



calcite, suggesting tliat the dolomitization may have been con- 

 temporaneous. 



It has been shown that in certain contemporaneous dolomites, 

 fossil structures, including corals, have resisted dolomitization to 

 a greater extent than the matrix has done owing to the non- 

 recrystallized material of the matrix being more unstable than the 

 more coarsely crystalline material of organic remains. 



(q) From this it may be inferred that the greater development of 

 dolomite in the matrix than in fossil structures, including corals, is 

 in favour of the theory of contemporaneous dolomitization.' But 

 Avhile this may be true in many cases, it is by no means certain that 

 selective phenomena of this kind should always be relied upon to 

 furnish conclusive evidence of the period of dolomitization. In 

 connexion with the question of the relative stability of matrix and 

 fossil structures, it must be remembered that the calcareous contents 

 of dolomitic limestones may have consisted originally of any or all 

 of the following forms of calcium carbonate : aragonite mud, more 

 coarsely crystalline aragonite, calcite mud, and more coarsely 

 crystalline calcite. Of these, aragonite mud is certainly the most 

 easily converted into dolomite, and coarsely crystalline calcite is the 

 most stable, but whether the more coarsely crystalline aragonite of 

 coral tissiies is more unstable than calcite mud appears to be an 

 open question. It is just this point which weakens evidence afforded 

 by selective dolomitization. In the contemporaneous alteration of 

 a coral limestone containing few other organic remains, the matrix 

 consisting mostly of aragonite mud would certainly be more easily 

 dolomitized than the coarser coral structures. 



Most organic limestones, however, consist largely of calcific 

 remains as well as coral structures, so that calcite mud must have 

 been present oi'iginally in the matrix, and may even have been in 

 excess of aragonite mud. If calcite mud is more susceptible to 

 alteration than coarser aragonite, in contemporaneous dolomitization 

 the matrix would still be dolomitized in preference to coral structures. 

 On the other hand, should calcite mud be more stable than coarser 

 aragonite, contemporaneous alteration of a mixed organic limestone 

 would result in the development of dolomite more in coral structures 

 than in the matrix. Again, there appears to be no means by which 

 the proportions of aragonite and calcite muds in the original matrix 

 may be ascertained. Considerations of this kind suggest that the 

 phenomena of the selective dolomitization of organic rocks can 

 scarcely be relied upon to supply sound evidence, particularly in 

 cases where such jDhenomena appear to contradict the evidence 

 afforded by field relations or other reliable features. 



Selective alteration of oolitic limestones may be more reliable, 

 ^ince the differentiation in such cases is between coarser calcite and 

 finer calcite. If dolomitization has attacked ooliths in preference 

 to a recrystallized matrix, the alteration is probably subsequent ; 

 on the other hand, the development of dolomite more in a non- 

 recrystaliized matrix than in ooliths may indicate contemporaneous 

 dolomitization, 



^ Sivansea (Mem. Geol. Sm-v.), p. 15. 



