Prof. Robert G. Wallace — Gypstim and Anhydrite. 271 



deposits appears at present to be very scanty, and further investigation 

 is greatly needed. 



EXPLANATION OF PLATES XX AND XXI. 



Plate XX. 



Fig. 1. Ortonella furcata, gen. et s^.no\. Section through a typical nodule. 

 Natural size. Base of the Seminula gregaria sub-zone, Eaven- 

 stonedale, Westmorland. 

 ,, 2. O. furcata. Thin section through above. x 20. 

 ,, 3, 4. O. furcata, showing dichotomous branching of tubes, x 55. 



Plate XXI. 



Fig. 1. Spongiostroma of. Malacostroma concentricum, Giirich. x 2-5. 



Base of the Seminula gregaria sub-zone, Fawcett Mill, near Orton, 



Westmorland. 

 ., 2. Mitcheldeania gregaria, Nich. x 26. Showing coarse and fine 



tubes. Zone of C1-C2 of Dr. Vaughan's classification. Scully 



Grove, Mitcheldean. 

 ., 0. Aphralysia carbonaria, gen. et sp. nov. x 13. Intergrown witli 



layers of a flocculent deposit — ' Spongiostrorna.'' 

 ,,4. A. carbonaria, gen. et sp. nov. x 30. From the 'Algal Band' 



associated with Ortonella. Base of Seminula gregaria sub-zone,. 



Wath, near Orton, Westmorland. 



IV. — Gypsum and Anhydkite in Genetic Relationship. 



By Egbert C. Wallace, M.A., Ph.D., D.Sc, F.G.S., 

 Professor of Geology and Mineralogy, University of Manitoba. 



(Published by permission of the Director of the Geological Survey of Canada.) 



IN certain geological horizons where there is evidence of continuous 

 deposition of salts from the waters of inland basins, gypsum and 

 anhydrite are found closely associated, while in other similar horizons 

 gypsum may occur with apparently no trace of anhydrite. Some 

 Upper Silurian occurrences in North America, the Lower Carboni- 

 ferous of Eastern Canada, and the Zechstein of Northern Germany are 

 illustrative of the association of both minerals, while in more recent 

 gypsum horizons anhydrite seems to appear less frequently. Eveu 

 in the one horizon anhydrite is found to be of only local occurrence. 

 In New York and in Micliigan the Salina formation contains both 

 gypsum and salt deposits, but practically no anhydrite. In Manitoba, 

 on the other hand, in an Upper Silurian formation which cannot be 

 definitely correlated with the Salina of New York, but which is at 

 any rate of approximately the same age, gypsum and anhydrite occur 

 in the most intimate relationship. 



The conditions of stability of gypsum and of anhydrite, whether in 

 presence of water or of salt solutions, were investigated by van't 

 Hoff and his co-workers by vapour-tension methods. Tlie relationship 

 of botb forms to the half-hydrate (CaS . ^HjO), whicli is obtained 

 by ordinary calcination processes, and to 'soluble anhydrite', which 

 is formed when precipitated gypsum is maintained at temperatures 



