and Leicestershire Dolomites. 247 



With regard to this classification it must be ncfted that as some 

 coufusiou has existed concerning the significance of the term 

 " contemporaneous", that name is used here strictly to denote 

 dolomites of secondary origin. 



The term " subsequent " has a wider significance than that of 

 " vein" doloraitization, as certain leached dolomites and some other 

 dolomitized rocks of undoubted subsequent origin cannot be described 

 adequately as vein dolomites, since some of them do not occur in 

 association with veins and channels. Evidently vein dolomites 

 constitute a subdivision of subsequent dolomites. It is now generally 

 admitted that the majority of dolomites are of secondary origin, the 

 contemporaneous class probably being more numerous than the 

 subsequent, but certain cases of dolomitization appear to be explained 

 most satisfactorily by the theory of primary deposition. Many 

 chemical experiments have been performed in the endeavour to 

 produce dolomite at different pressures and temperatures, but with 

 limited success.^ In spite of the comparative failure of these 

 experimental attempts to produce dolomite artificially, the fact that 

 it does occur in nature as a chemical precipitate is shown by its 

 occurrence in mineral veins. It is very doubtful Avhether any 

 reliable evidence can be obtained from experiments performed under 

 conditions which may be quite unlike the natural conditions which 

 existed in the seas of remote geological periods. 



In determining the class to which a dolomite belongs, one must 

 rely upon the collective evidence afforded by : — 



Pield relations, 

 Inherent structural features, 

 Selective dolomitization, and 

 The absence or presence of fossils. 



Space permits only an incomplete survey of the phenomena 

 connected with dolomitization, and a short discussion of the more 

 reliable sources of evidence is all that is attempted. 



2. Field Evidences. 



{a) The occurrence of truly bedded dolomites associated with beds 

 of such deposits as gypsum or rock salt is considered to support the 

 view that the dolomite was primarily precipitated. - 



{h) The theory of primary deposition is also supported by the 

 occurrence of genuine beds of dolomite alternating with beds of 

 limestone. In this case it is inferred that the dolomite was either 

 chemically precipitated,* or laid down as a clastic deposit derived 

 from a source different from that of the non-dolomitic limestone. 

 Pseudo-interbedding, characterized by the failure of the dolomitization 

 to conform accurately to bedding planes, the lateral transition of 

 dolomite into limestone, and sometimes by a streaky development of 



1 See F. W. Clarke, "Data of Geo-chemistry " : Bull. Geol. Surv. U.S.A., 

 No. 616, p. 559, 1916. 



- See Weigelin, NeuesJahrb., Beil. Bd. xxxv, p. 628, 1913. 



^ Suess, The Face of the Earth, English translation, vol. ii, p. 262, 1906. 



