202 MR. B. SMITH ON THE [vol. lxXlV r 



1912. Rogers, A. F. — ' The Occurrence & Origin of Gypsum & Anhydrite at the 

 Ludwig Mine, L} r on County, Nevada' Economic Geologj', vol. vii, pp. 185- 

 89 ; and discussion by J. Claude Jones, ibid. pp. 400-402. 



1914. Wallace, R. C. — ' Gypsum & Anhydrite in Genetic Relationship ' Geol. 



Mag. dec. 6, vol. i, pp. 271-76. 



1915. ' Special Reports on the Mineral Resources of Great Britain ' Mem. Geol. 



Surv. Vol. iii, ' Gypsum & Anhydrite ' [2nd ed. 1918]. 



1916. Clarke, P. W. — ' L)ata of Geochemistry ' 3rd ed. Bull. U.S. Geol. Surv. 



No. 616. 

 1916. H. Ries & T. L. Watson. — 'Engineering Geology' 2nd ed. 

 1916. Smith, B. — 'Ball or Pillovvform Structures in Sandstones' Geol. Mag. 



dec. 6, vol. iii, pp. 146-56. 



EXPLANATION OF PLATES XVII & XVIII. 



Plate XVII. 



A pillar of gypsum, Chellaston Alabaster Quarries. This shows the upper, 



fairly-continuous, white alabaster, overlying pink or red alabaster with 



included masses of white — forming the ' Breccia.' The nearly horizontal 

 markings are wire-saw cuts. (See p. 178.) 



Plate XVIII. 



Fig. 1. Section showing the contact between the top of the anhydrite and 

 the overlying seam of gypsum at Cocklakes Mine, Cumberland. On 

 the left of the slide is anhydrite with veins of gypsum ; on the right 

 of the slide is gypsum enclosing isolated patches of anhydrite. The 

 section shows, at the top, pseudomorphs of gypsum after anhydrite. 

 The small black rhombs are dolomite. Ordinary light. Magnification 

 X 16. (See p. 196.) 



2. The same as above, between crossed nicols. The points marked A in 



the two sections are identical. Magnification X 16. (See p. 196.) 



3. Section showing the contact between the bottom of the anhydrite 



and the underlying seam of gypsum at Cocklakes Mine, Cumber- 

 land. On the left of the slide is anhydrite ; on the right of the slide 

 gypsum. The irregular (' moth-eaten ') boundary of the anhydrite 

 is seen, where the crystals lie transversely to the average line of 

 contact with the gypsum. Isolated patches of anhydrite are partly 

 replaced by brown-stained gypsum. The small black rhombs are 

 dolomite. Ordinary light. Magnification X 25. (See p. 197,) 



Discussion. 



The President (Dr. A. Harker) expressed his appreciation of 

 the value and interest of the paper, and remarked that each 

 occurrence of gypsum and anhydrite offered its own problems. 



Dr. J. W. Eyans thought that the Author had brought forward 

 a number of interesting new observations with regard to the struc- 

 ture and relations of the deposits described. He enquired whether 

 these were not mostly of Triassic age. The mineral precipitates of 

 the Permian were apparently the result of the desiccation of an 

 isolated marine area at a temperature, as a rule, sufficiently high to 

 determine the formation of anhydrite, which has since been con- 

 verted into gypsum. This has been shown to be the case in 

 Germany, and the course of events was doubtless the same in the 



