Br. D. Woolacott — Magnesian Limestone of Durham. 455 



are really mixtures of dolomite and calcium carbonate in varying 

 proportions. Analysis of this rock have been published giving 

 a higher percentage of magnesian carbonate than the dolomitic 

 ratio, viz. 45-7. * I always thought these wrong and misleading, and 

 the work of Dr. Trechmann on the composition of these rocks has 

 definitely proved them to be so. 2 The best way, therefore, to 

 state their analyses for geological work is to give, as Trechmann has 

 done, the percentage of dolomite and calcite, together with the 

 smaller amounts of other substances present. The relative proportion 

 of dolomite and calcite in the rock along with the original presence, 

 of calcium sulphate has probably had, as I shall endeavour to show 

 most important influences on the nature of the chan ges — segregational, 

 concretionary, etc. — that have taken place on such a marked scale in 

 these rocks. The presence of impurities has also had an effect on the 

 structural changes, e.g. the presence of argillaceous and siliceous 

 impurities appears to have deterred the formation of the concre- 

 tionary structures, and Dr. Trechmann has suggested that the 

 dolomitization of certain parts of the lleef has been arrested by the 

 occurrence of manganese dioxide. 



The original presence of calcium sulphate in the Permian rocks is 

 not now a matter of conjecture. In 1875 Dr. T. Sterry Hunt 3 and 

 in 1881 Wilson 4 suggested that calcium sulphate was a subsidiary 

 product of the deposition of dolomite and it has been known for 

 many years that gypsum and anhydrite were associated with the 

 Permian rocks. These substances are found in Permian strata 

 in the Vale of Eden, 5 near St. Bees Head, and in the Zechstein 

 of Germany, and have been proved to occur in the Magnesian 

 Limestone of South Durham, Yorkshire and North Lincolnshire. 6 



The precipitation of calcium sulphate would appear to have been 

 a product of the deposition of dolomite and dolomitic limestones, 



1 Analysis by Browell and Kirkby, Nat. Hist. Trans., vol. i, pt. xi, p. 204, 

 1866. 



2 Q.J.G.S., vol. lxx, p. 232, 1914. 



3 Chemical and Geological Essays, 1875. 



4 " The Permian Formation of N.E. England " : Mid. Nat., vol. v, p. 202, 

 1881. 



5 Eegarded as Trias by some geologists. 



6 Anhydrite and gypsum have been proved by borings in the Permian at 

 Seaton Carew (Wilson, Q.J.G.S., vol. xliv, p. 781, 1888) ; Whitehaven, near 

 Norton (Tate, Q.J.G.S., vol. xlviii, p. 488, 1892) ; Hartlepool (Trechmann, 

 Q.J.G.S., vol. lxix, p. 184, 1913); Leeming Lane; near Stoekwith, North 

 Lincolnshire (G. Dunston, Fed. Inst. Min. Eng., vol. xii, p. 578, 1896-7) ; in 

 borings in South Yorkshire. Concealed coalfield of Yorkshire and Nottingham 

 (Walcot Gibson, Mem. Geol. Surv., 1913) ; at Market Weighton (Walcot 

 Gibson, Summary of Progress, Geol. Surv., p. 43, 1917). Trechmann gives 

 the following analysis of the Seaton Carew boring (Q.J.G.S., vol. lxix, p. 193, 

 1913) :— 



Feet. 

 Sulphate-free limestone . . . . . . 480 



Limestone impregnated with gypsum and anhydrite . 358 

 Pure anhydrite and gypsum ...... 35 



Total thickness of magnesian limestone . 878 



The mass of anhydrite and gypsum at Hartlepool was 267 ft. 2 in. It was 

 overlaid by glacial deposits, so it may have been originally much thicker. 



