248 L. M. Parsons — Dolomitization 



dolomite ia the intervening limestone, is in favour of siibsequent 

 dolomitization.^ 



(c) The presence of interbedded conglomerates containing fragments 

 of dolomite derived from older dolomites below would suggest that 

 the dolomitization of the older beds was not of subsequent origin, 

 and that the dolomitic material of the conglomerate was a primary 

 clastic deposit.^ 



(d) The persistence of uniform bedded dolomites over a wide area 

 without lateral transition into unaltered or poorly magnesian lime- 

 stones is considered to be one of the strongest evidences in favour of 

 contemporaneous dolomitization.^ This conclusion receives further 

 support if such dolomites occur at the same stratigraphical horizons 

 in different areas, but the presence of dolomite at a certain horizon in 

 one area and its absence at the same horizon in another area, does 

 not necessarily indicate dolomitization of subsequent origin. Dolomite 

 may have been formed in shallow water, while poorly dolomitic 

 or unaltered limestones were being deposited further from the 

 shore-line.* 



{e) Should beds of dolomite be found, when traced laterally, to 

 pass into unaltered or poorly magnesian limestones, the inference is 

 in favour of subsequent dolomitization,^ provided that other more 

 conclusive evidence of a different origin is not forthcoming. 



(/) A patchy development of dolomite and limestone due to rapid 

 lateral transition from one into the other is a modification of («), and 

 lends support to a similar conclusion. * A patchy development of 

 dolomite and limestone on a small scale, known aspseudo-brecciation, 

 is discussed later. 



(ff) The evidence in favour of subsequent dolomitization is much 

 stronger when an irregular or patchy development of dolomite is 

 associated with faulting or jointing. In such cases it is obvious 

 that the fault planes and joints have probably served as channels for 

 percolating magnesian waters of a period subsequent to that during 

 which the rock was originally deposited.' Dolomites of this class 

 are properly described as vein dolomites. 



(h) Faulting associated with extensive and non-patchy dolomites 

 (<?) appear to indicate that the faulting occurred after dolomitization. 



While discussing field relations we may observe that great thickness 

 of bedded dolomite has been considered to support the view that the 

 dolomite was of primary origin, but since a thick mass of limestone 

 may be dolomitized during a long period of subsidence,* as in the 

 case of certain coral reefs, it is questionable whether mere thickness 



^ Calvm, Iowa Geol. Surv., vol. vii, p. 151, 1896. 



- See Swansea (Mem. Geol. Surv.), 1907, p. 13. 



^ Dixon, S^uansea (Mem. Geol. Surv.), 1907, p. 13. 



* Id., p. 15. 



'' Hardman, Proc. Eoy. Irish Acad., ser. Il, vol. ii, p. 728, 1875-7. 



^ F. M. Van Tuyl, " The Origin of Dolomite " : Iowa Geol. Surv., vol. xxv, 

 p. 364, 1916. 



■^ The Geology of the SouthWales Coal Fields (Mem. Geol. Surv.),pt. ii, p. 33. 



^ Skeats, " On the Dolomites of the Southern Tyrol " : Q.J.G.S., vol. Ixi, 

 p. 97, 1905. 



