Section IV., 1913. [139] Tr.\ns. R.S.G. 



A Contribution to the Study of Dolomitization. 



By R. C. Wallace, University of Manitoba, Winnipeg. 



Presented by Dr. A. H. R. Buller, F.R.S.C. 



(Read May 29, 1913) 



INTRODUCTORY. 



A recent investigation carried out by the writer into the cause of 

 the mottling of certain Ordovician limestones in Manitoba* has raised 

 some points of interest in relation to the much discussed question of 

 dolomitization. The limestones are characterised by irregular markings 

 of a dark brown or slightly bluish colour, which are found to represent 

 recrystallised areas with a much higher magnesia content than that of 

 the grey setting in which they are placed. The dolomitization was in 

 all probability a contemporaneous phenomenon, taking place while 

 the calcareous ooze was still in the process of hardening, and is not to 

 be ascribed to magnesia-bearing waters effecting the transformation as 

 they percolated downwards through already hardened limestones. The 

 Mg salts were most probably derived from decomposing algae, which 

 became buried in the gradually accumulating ooze at the bottom of a 

 quiet sea of only moderate depth. The increase of Mg ions in the 

 waters which permeated the ooze in the immediate vicinity of the algae 

 gave rise to a purely local mineral transformation. The deepening in 

 colour is due to the presence, in the dolomitized areas, of haematite and 

 limonite, which are to be ascribed to the same source as the salts of 

 magnesium. 



Two facts arising out of this investigation should be emphasised. 

 The first is that the mineral transformation has taken place under con- 

 ditions of temperature and pressure which have effected no recrystal- 

 lisation in the surrounding limestone : and in this connection it must be 

 borne in mind that the process has not taken place at the surface of 

 the sea. The second is that although microscopic examination proves 

 the darker material to be composed of homogeneous crystals which react 

 for dolomite optically and with Lemberg's solution, the proportion of 

 MgCOg to CaCOg was found on analysis to be less than 1 : 3 (MgCOg 

 23,35% CaCOg 71.03%). In the voluminous literature which has grown 

 up round the question of dolomitization, these two points may be 



* Journal of Geology, XXI, 5, 1913, p. 402. 



Sec. IV., 1913-9 



