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



from a dolomitic rock to a highly compressed, slickensided, compact, 

 calcareous breccia. 1 The change here has been similar to that in 

 the case of the Flexible Limestone. In Fox Cover Quarry, Seaham 

 Harbour, in rocks apparently not affected by pressure, a large mass 

 of the Reef has been changed by solution from a dolomite into 

 a nearly pure limestone. The dolomite can be seen to pass gradually 

 into the calcareous rock, and when the alteration is complete the 

 fossils are entirely obliterated. Other altered rocks and breccias of 

 a similar nature occur. These changes have not been brought about 

 by the mechanical removal of dolomite from the magnesian limestone, 

 but by solution. There would appear to me to be two methods of 

 explaining this: (1) the dolomite has been dissolved and meta- 

 somatically replaced by calcite, or (2) the magnesium carbonate has 

 been carried away as a soluble salt out of the dolomite, calcium 

 carbonate metasomatically replacing it. 



(1) That direct solution of dolomite may have gone on in a small 

 degree is undoubted, but I do not think that it has taken place on 

 any large scale in the magnesian limestone, and am of the opinion 

 that the carrying away of the magnesium as a soluble salt is in the 

 main the explanation of these altered rocks. If these changes were 

 brought about by direct solution of dolomite, more of this mineral 

 would occur in geodes, as such a slightly soluble substance (in 

 comparison with calcite) would be redeposited in other parts of the 

 rock if the pressure conditions were the factor determining its 

 solution. Another significant fact is that most of the altered lime- 

 stones originally contained a very small percentage of interstitial 

 calcite. It is also difficult to conceive of a solution dissolving 

 dolomite under these conditions and at the same time containing 

 calcite to replace it — if it was not immediately replaced owing to 

 the small percentage of interstitial calcite the rock would have been 

 broken up and the strata disturbed. Alteration of the Flexible 

 Limestone occurs, however, without any disturbance, and the 

 brecciation of other altered rocks was brought about by thrusting 

 and is not directly due to solution changes. In the solution of 

 magnesium carbonate from dolomite in the presence of calcium 

 sulphate, calcium carbonate is produced in the reaction, and there is 

 besides the interstitial calcite and calcite of the dolomite. 



solution of almost the entire rock and its replacement by calcite, e.g. Flexible 

 Limestone, Marsden, interstitial calcite 0-71. The analyses given by 

 Dr. Trechmann of this bed above the thrust plane at Hendon are — 



Flexible Hard Calcareous Eock 



Limestone. (altered Flexible). 



MgCOs . . 43-78 10-92 



CaC0 3 . . . 55-01 88-56 



Fe 2 3 . . . 0-61 0-19 



Insoluble residue . 0-88 10-92 



Dolomite . . 95-91 23-93 



Calcite . . . 2-88 75-55 



1 This section is drawn and described in my paper on "The Permian of 

 N. Durham ", op. jam cit., fig. 10. 



