462 Dr. D. Woolacott — Magnesian Limestone of Durham. 



Sorby suggests that probably the alteration of some magnesian 

 limestones was affected by the percolation of magnesium salts, and 

 adds: "a process the very reverse of that just described is now 

 taking place by the action of dissolved gypsum, by which sulphate 

 of magnesia, frequently efflorescing on the surface of the rock, and 

 carbonate of lime are produced" (Reports Brit. Assoc. Sci., p. 77, 

 1856). This geologist had evidently arrived at the same conclusion 

 as I have from direct observation in South Yorkshire regarding the 

 action of gypsum on dolomite. 



(If it is objected that the calcium sulphate solutions were all 

 removed from the rock at the time of thrusting, it must be 

 remembered that in the natural waters contained in the rocks there 

 would be other dissolved substances, e.g. common salt, etc., which 

 may have had an influence in bringing about the solution of the 

 magnesium, and that under the high pressures existing at that time 

 — which I have endeavoured to prove reached in the Marsden area 

 a pressure of some 300 tons per square foot — the solution effects of 

 carbonic acid would be greatly increased. I, however, see no reason 

 for doubting that the main factor for the solution of magnesium 

 carbonate from dolomite was the presence of calcium sulphate 

 solution in the rock.) 



The dissolved magnesium salt may have dolomitized other parts 

 of the limestone in regions of less pressure and lower temperature, 

 or where the concentration was different, or have been carried 

 entirely away in the same manner as the calcium sulphate. 



It should be noticed that in this type of demagnesified rock, 

 where the alteration is complete owing to the non-production of 

 a cellular texture, no mechanical removal of dolomite is taking or 

 has taken place. 



2. The concretionary limestones were originally dolomitic lime- 

 stones, and there can be little doubt that these rocks were impregnated 

 with sulphates and that the solution of these brought the bed into- 

 a condition which gave the concretionary forces full play ; and 

 further it would seem highly probable that the complicated structures 

 occurring in these rocks were mainly produced when the beds were 

 saturated in solutions of calcium sulphate containing colloidal 

 organic matter. 1 It is the presence of this latter material that causes 

 the spheroidal condition. Colloids readily permit the diffusion of 

 crystalline salts through them. The effect of colloids on calcium 

 carbonate in producing concretionary growths has been proved by 

 Rainey, Harting, Ord, and Stocks. 2 I have before referred to the 

 presence of colloidal matter as a factor in causing the concretionary 

 limestone, 3 and Trechmann has also suggested that the spherical 

 concretions are due to the presence of organic matter when they 

 were forming. 4 The Brvozoa lleef probably formed the reservoir 

 from which the organic matter came. I have divided the concretions 



1 The concretionary limestones often contain organic matter and are some- 

 times strongly fetid. 



2 Stocks, Q.J.G.S., vol. lviii, pp. 46-58, 1902. 



3 Univ. Durham Phil. Soc. Proc, vol. iv, pt. v, p. 271, 1911-12. 



4 Q.J.G.S., vol. lxix, p. 198, 1913. 



