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



calcium sulphate forming and being deposited when the solution 

 became saturated. In. areas where the magnesian limestone is still 

 covered by a mantle of later rocks the sulphates remain intact, but 

 from this limestone in the greater part of Durham they have been 

 removed. Trechmann's analyses show that many of the limestones 

 in Durham (in areas where the beds and intercalations of gypsum or 

 anhydrite have been removed) contain a small percentage of 

 sulphates, and lately crystals of gypsum have been found in geodes 

 in the lower limestone at Raisby Hill Quarry in South Durham. 

 The evidences of the former existence of sulphates is to be found in 

 the letting down of beds, 1 in some of the extensive disturbance and 

 brecciation of the strata, in the hollow oolites of Roker and 

 Hartlepool, and in cavities left by crystals and crystalline aggregates. 

 Indirectly their solution had most important influences, not only did 

 it render the rock highly cavernous, cellular, and porous, but at the 

 time of its removal the strata would be saturated with solutions of 

 calcium sulphate, which appear to have had marked effects on 

 certain parts. As I shall endeavour to show, the stewing of the 

 magnesian limestone in warm solutions of calcium sulphate, some- 

 times under high pressure, has been the factor that has altered large 

 parts of it into a more calcareous rock. 



The conditions in which the magnesian limestone was deposited 

 were those of an inland sea containing various salts in solution. 

 According to chemical theory these salts would be existing as ions, and 

 so the solution would contain Ca", Mg", K', Na', H 2 CO s ", SO"*, CI'. 

 When in equilibrium various compounds would be existing, such as 

 CaCOs, CaCl 2 , MgCO s , MgS0 4 , rTaCl, Na 2 SC>4, etc., together with 

 disassociated ions. Owing to varying conditions of temperature, 

 pressure, and concentration, there would be a tendency for the 

 formation of the least soluble and most soluble salts, thus calcium 

 carbonate and magnesium sulphate would be formed. The magnesium 

 sulphate afterwards acted on part of the calcium carbonate to 

 form either free magnesium carbonate or the double salt, dolomite. 

 If free magnesium carbonate were deposited, then the two separately 

 precipitated carbonates must at a later stage have combined to 

 form dolomite. Calcium sulphate would be formed as a subsidiary 

 product, and deposited according to the concentration and temperature. 

 Gypsum or anhydrite were deposited with the variation of conditions 

 in thick beds, in veins, in irregular pockets, 2 or were irregularly 

 intercalated throughout the limestone. 3 



1 A considerable thickness of limestone (some 200 feet) appears to be absent 

 on the west side of Cleadon Hills, and it may be that a thick bed of sulphate 

 has been removed here. The extensive slipping down of beds and consequent 

 brecciation along the eastern side of the Eeef, which can be seen at several 

 places along the Durham coast, is most probably due to this cause. Trechmann 

 has noted several vertically slickensided surfaces (Q.J.G. S., vol. lxix, p. 201). 



2 Geodes containing gypsum with dolomitic rhombs disseminated through 

 them have been observed by Trechmann. 



3 Anhydrite was deposited in periods of drought and higher temperature. 

 See discussion of this subject byB. Smith in "The Chellaston Gypsum-breccia 

 and its relation to the Gypsum-anhydrite deposits of Britain": Q.J.G.S. , 

 vol. lxxiv, p. 195. 



