264 ME. C. T. TBECHMATTSr OX THE LITHOLOGT [June 1914, 



Fig. 3. Shell- Limestone. Easington Colliery knoll. Analysis XXXI. Bryo- 

 zoonal Reef. The bryozoa are completely dolomitized. In the cells 

 occur aggregates of clearer calcite and dolomite. The matrix is 

 dolomitic with a few calcareous grains, and is highly contaminated 

 by dirt-inclusions. X 11. (See p. 240.) 



4. Calcareous Shell- Lime stone. Tunstall Hill. Analysis XXXVII. A 



portion of the reef which has escaped dolomitization, though a few 

 rhombs of dolomite are seen locally. The slide is made up of sections 

 of bryozoa, brachiopoda, etc., in a mosaic of calcite. The darker 

 portions are due to the presence of carbonaceous and manganiferous 

 material. X 4. (See p. 241.) 



5. The Great Marl-Bed. Fulwell Quarries. Analysis XVI. Calcite and 



dolomite-grains, some of the latter developing rhombic outlines. 

 The calcite tends to separate out along lines apparently of strain or 

 weakness. X 7. (See p. 237.) 



Plate XXXVII. 



Fig. 1. Anhydrite and dolomite from the base of a mass of anhydrite. Warren 

 Cement- Works Boring, Hartlepool. Analysis XLIX. The anhydrite 

 occurs as radiating stellate aggregates, in a hard, compact, dolomitic 

 matrix. X 10. (See p. 243.) 



2. Oolite. Sea-coast section near Castle-Eden Dene. The concentrically- 



arranged material is dolomite, and each grain is filled with an 

 aggregate of brown allotriomorphic fluorite-crystals, probably of 

 secondary origin. X 7. 



3. Oolite in the Upper Limestones. Hardwick Dene. Dolomitic oolites 



with centres formed of calcitic aggregates, lying in a very loosely- 

 compacted dolomitic matrix. The light portions are original spaces. 

 X 14. 



4. A large - grained oolite associated with Upper Shell - Limestone. 



Analysis XXVH. Blackhall Bocks. The concentrically-arranged 

 material is nearly pure dolomite. Both the nuclei and the matrix 

 are partly dolomitic, and partly calcitic. X 2*5. (See p. 239.) 



5. Oolitic bed in the Upper Limestones. Seaham Harbour. The larger 



spaces are stellate cavities enlarged and obscured in the process of 

 grinding the slice. Analysis VI. A fine-grained dolomitic oolite, 

 the centres dissolved out and the concentric structure almost 

 obliterated through recrystallization of the dolomite in small 

 allotriomorphic grains. X 7. (See p. 235.) 



Discussion. 



Dr. J. W. Evans expressed his admiration of the Author's 

 careful and detailed work, that formed the foundation of the paper 

 and was in agreement with the conclusions at which he had arrived. 



He objected to the use of the word dedolomitization in con- 

 nexion with the Magnesian Limestone. The term was originally 

 employed for the process by which, in the course of thermal 

 metamorphism, the magnesia was removed from a dolomite and 

 converted either into periclase or into a magnesium silicate, leaving 

 behind pure carbonate of lime. In the case of the Magnesian 

 Limestone there was absolutely no evidence that magnesia had 

 anywhere been abstracted from dolomite. The removal of dolomite 

 in the form of powder was a different process, similar to any other 

 process by which dolomite was eroded without decomposition, and 



