458 Br. D. Woolacott — Magnesian Limestone of Durham. 



parts have been decalcified by manganese or other solutions, 1 but the 

 most interesting alteration is, as I stated in my paper some years 

 ago, that large areas and considerable thicknesses of the magnesian 

 limestone of Durham are more calcareous than they were originally. 

 This latter change has been brought about by solution and also by 

 the mechanical removal of dolomite. I first referred to it in my 

 paper on the Permian of North Durham as " dedolomitization", 

 a term the use of which in this connection was objected to by 

 Garwood and Evans, as it has been used by Teall and others in 

 another sense. If, however, the alteration of a limestone into 

 dolomite is "dolomitization", then surely the alteration of the latter 

 rock into a calcareous one is the simplest case of dedolomitization. 

 As long as it is recognized that large masses of the magnesian 

 limestone are more calcareous than they were originally, I have no 

 desire to use any term, but as some word would be useful (if 

 dedolomitization is inadmissible) I would suggest " demagnesification" 

 for the processes — solution and mechanical — by which parts of this 

 rock have been rendered more calcareous. As I shall show, 

 <( demagnesified" beds, both due to solution and mechanical processes, 

 are of wide occurrence and thickness. The mechanical removal of 

 dolomite by running water from the magnesian limestone has been 

 admitted by Professor Garwood, Dr. Evans, and Dr. Trechmann. It 

 is obvious as a result of the setting free of dolomitic powder in the 

 rocks in which the concretionary and segregated processes have taken 

 place. Objection has, however, been taken to my views regarding the 

 solution of magnesian carbonate out of dolomite. Dr. Evans has 

 asserted that "there is in the case of the Magnesian Limestone 

 absolutely no evidence that magnesia has anywhere been abstracted 



4 Parts of the Bryozoa Reef have been dolomitized, e.g. Trechmann records 

 the shell substance of a Productus as containing 95-88 per cent of dolomite, 

 and of an Area 98-68. Much of the sediment which fell on the reef was 

 calcareous, and there is little doubt that the highly dolomitic nature of parts 

 of the reef is due to subsequent dolomitization. This may have gone on pene- 

 contemporaneously with deposition by the magnesium waters of the sea, lime 

 being removed and replaced by magnesia, in the same way that the coral reefs 

 of the present day have been proved to undergo a process of dolomitization 

 (Judd, " Atoll of Funafuti," Royal Soc. London, 1904). Certain observations 

 on the decalcified rocks in the reef, however, lead me to think that 'the 

 dolomitization of parts of it may have taken place long after deposition. It is 

 possible, if the solution changes which are discussed in this paper have taken 

 place, that the magnesium may have been transferred from one part of the 

 limestone to another. This appears to be so in the case of parts of the reef, 

 and in some of the dolomitized highly calcareous rocks known as "bluestones" 

 both in the upper and lower limestones. I have lately examined breccias 

 occurring in the lower calcareous limestones of Raisby Hill Quarry which 

 have been formed by the alteration of these rocks by magnesium and other 

 solutions (see Breccias, Part II of this paper). 



1 Portions of the reef have been partially decalcified by manganese solutions, 

 Tunstall Hill, Fox Cover Quarry, Beacon Hill. Trechmann gives an analysis 

 of a decalcified rock from Tunstall Hill — 



