Vol. 70.] OF DURHAM MAGNESIAN LIMESTONES. 233 



I was also desirous of gaining information, if possible, regarding 

 the original condition of deposition of the formation, by analysis of 

 those parts which had undergone little, or relatively little, subsequent 

 alteration ; and I also wished to ascertain to what extent those 

 portions which have a calcareous composition might be regarded 

 as the result of original conditions of deposition, of escape from 

 secondary dolomitization, or of subsequent calcareous segregation. 



The advisability of further analytical investigation was recently 

 emphasized by some significant remarks, which Prof. E. J. Garwood 

 and Mr. E. E. L. Dixon made during the discussion of a paper 

 by Dr. D. Woolacott. 1 Among the questions asked during this 

 discussion were the following : — 



(a) What precisely was the agency which deprived a dedolomitized lime- 



stone of its magnesium ? 



(b) What was the evidence that any of the dolomite was an original 



chemical precipitate ? 



The only extended series of analvses of English Magnesian 

 Limestones is that of E. J. J. Browell & J. W. Kirkby. 2 The 

 specimens were selected by Kirkby, and analysed in Browell's 

 laboratory. A critical examination of this paper shows that the 

 analyses are mainly concerned with Marl Slates, Lower Limestones, 

 and the concretionary growths and their associated matrix in the 

 Upper Limestones of Fulwell. 



The authors find that their analyses indicate variations from a 

 rock containing 96'94 per cent, of calcimn carbonate and 1*66 of 

 magnesium carbonate, to one containing 42 '84 per cent, of the 

 former and 49'86 of the latter. At least six out of their forty-five 

 analyses give quantities of magnesium carbonate in excess of the 

 dolomitic ratio. The chief conclusion arrived at by them is that 



' the differences in chemical composition are largely due to the action of 

 segregation after deposition,' 



of which the most conspicuous example is cited — the botryoidal and 

 cannon-ball concretions. They also conclude that 



' the light-coloured, friable beds contain the most magnesia, while the dark 

 compact beds are the most calcareous,' 



a conclusion only applicable to the sulphate-free and oxidized rocks 

 as they appear at the surface, and not to the formation in its more 

 original condition — as proved in sub-Triassic borings or in other 

 protected situations. They also remark upon the smallness of the 

 insoluble residues. 



There can be no doubt that the bewildering inconstancy in the 

 dolomitic character of the analyses obtained by Browell & Kirkby 

 is largely due to an indiscriminate choice of partly-segregated 

 rocks. 



1 Q. J. G. S. vol. lxvii (1911) p. 314. 



2 ' On the Chemical Composition of Various Beds of the Magnesian Lime- 

 stone & Associated Permian Rocks of Durham ' Nat. Hist. Trans. North. & 

 Durham, vol. i, pt. 2 (1866-67) p. 204. 



r2 



