Dr. D. Woolacott — Magnesian Limestone of Durham. 459 



from dolomite ". 1 The evidence can be clearly studied in the field, 

 and a discussion of it is given in the following pages. Dr. Trechmann 

 admits the solution of dolomite, and while saying that he finds no 

 evidence for the leaching out of magnesian carbonate from dolomite, 

 yet adds "the question as to whether in the presence of saturated 

 solutions of calcium sulphate the dolomite was as stable as it is under 

 present conditions remains a problematic one." 2 I think that the 

 evidence in the field is clear that under certain conditions the magnesian 

 carbonate in solution has been carried away as a soluble salt from 

 the dolomitic limestones is clear. The important point, however, is 

 not the manner in which these changes were produced, but the 

 the fact that large portions of these rocks — both cellular and non- 

 cellular — are more calcareous than they were originally, and that 

 the chief cellular tj T pes of the limestone were not produced by 

 dolomitization. 



There are five rock types occurring in these limestones which have 

 a less content of magnesian carbonate than they had. They are 

 (1) dolomitic rocks and breccias altered into calcareous rocks chiefly 

 in regions of pressure at the time of thrusting without the production 

 -of a cellular structure, (2) the concretionary limestone, (3) the 

 segregated limestone, (4) the "fractured cellular" rocks, and (5) 

 the "negative breccias". The last four usually result in the 

 production of a highly cellular calcareous rock. As each of these 

 rocks present certain characteristic features, it may be well to 

 consider them separately. 



(1) In the area affected by the thrusting and locally at other 

 places, parts of these dolomitic limestones have been rendered more 

 calcareous without the production of either a cellular, concretionary, 

 or segregated structure. These changes are mainly found in the 

 area affected by the thrust-movements and would appear to be due 

 to solubility-conditions set up in the rock in the regions of pressure 

 and heat developed at that time. These alterations are best seen in 

 the dolomitic rocks above and beneath the thrust plane at Hendon. 

 Sunderland, and in the mass of highly slickensided calcareous breccia 

 (originally a dolomitic limestone) forming Jean Jiveson's Rock south 

 •of this thrust plane and probably connected with it, and in other 

 areas. The dolomitic Flexible Limestone at Hendon becomes a 

 hard, non-cellular, calcareous rock, and then a calcareous brecciated 

 mass on the underside of the thrust plane, the change taking place 

 within 100 feet and being easily traceable. 3 



The breccia of Jean Jiveson's rock occurs in another area of 

 pressure relief, and it is here possible to* trace the gradual change 



1 Q.J.G.S., vol. Ixx, p. 264. 



2 Ibid., p. 253. 



3 This section is noticed in my paper on "The Permian of N. Durham", 

 op. jam. cit., p. 267, 1912, and is drawn and described in fig. 10, where I refer 

 to the change as having been brought about by the leaching out of the 

 magnesium carbonate from the dolomite. Dr. Trechmann also fully describes 

 it (Q.J.G.S., vol. Ixx, p. 252, 1914), and explains the alteration as being due 

 to the solution of dolomite. The amount of interstitial calcite in the Flexible 

 Limestone is usually very small, so that solution of dolomite demands the 



