Dr. D. Woolacott — Magnesian Limestone of Durham. 497 



The most widespread breccias are the autoplastic breccias in the 

 north of the county, the broken-up beds due to the removal of the 

 sulphates, and the pseudo-breccias. It is a very noticeable feature of 

 many of the breccias that they are more calcareous than the dolomitic 

 limestones from which they were formed. 



Summary. 



The main conclusions that have been arrived at by recent work on 

 the Magnesian Limestone of Durham are : — 



1. That the series of bedded dolomites, dolomitic limestones, and 

 calcareous beds which form the main mass of these rocks are the 

 result of chemical precipitation, and that they originally contained 

 thick and thin beds, veins, pockets, and impregnations of anhydrite 

 and gypsum. 



2. That the main mass of the dolomite and dolomitic limestones 

 was either precipitated as a double salt or formed by the combination 

 of the associated carbonates after precipitation, and that it is possible 

 that examples of both of these methods of formation occur. 



3. That the Bryozoa Reef and small beds and lenticles in the 

 Lower and Upper Limestones were organically formed, and consist of 

 remains of organisms on which dolomitic or calcareous sediment 

 settled. 



4. That calcium sulphate appears to have been a subsidiary 

 product of the deposition of dolomite and dolomitic limestone. 



5. That while the main mass of the dolomite was chemically 

 precipitated some parts, e.g. parts of the reef, etc., may have been 

 dolomitized pene-contemporaneously with deposition and others 

 long after. 



6. That the Middle Magnesian Limestone exists under three 

 distinct facies, consisting of a Bryozoa Reef and its bedded in-shore 

 and off-shore equivalents. 



7. That (except in the case of parts of the reef, etc.) the cellular 

 structures in the Magnesian Limestone were not produced by 

 subsequent dolomitization. 



8. That parts of the Magnesian Limestone are more calcareous 

 than they were originally; the non-cellular type of these " cle- 

 magnesified rocks" occurs principally within the zone of thrusting, 

 while the cellular types are the highly altered concretionary, 

 segregated, and other rocks. 



9. That the chief non-cellular " demagnesified rocks" and breccias 

 were probably produced by the action of solutions of calcium sulphate 

 on the dolomite, the magnesium being dissolved out as a sulphate and 

 metasomatically replaced by calcium. 



10. That the concretionary rocks may have been produced in 

 solutions of calcium sulphate containing colloidal organic matter, 

 the more definite spherical types being formed in regions where the 

 organic matter was concentrated. 



11. That the calcareous segregated rocks may have been formed in 

 sulphate solutions free from organic matter. 



12. That solution of part of the magnesium-content of the 

 concretionary and segregated rocks has probably gone on, and that 



DECADE VI. — VOL. VI. — NO. XI. 32 



