174 Mil. B. SMITH ON THE [vol. lxxiv, 



7. T/><? CnELLA.STON GrYPSUM BRECCIA ill its IvELATTON to tlie 



Gypsum-Anhydrite Deposits of Britain. By Bernard- 

 Smith, M.A.., F.G.S. (Read December 19th, 1917.) 



[Plates XVII & XVIII.] 



Contents. 



Page 

 I. Introduction 1 74 



II. The Chellaston Breccia and Associated Sediments... 177 



III. Gypsum Deposits of other Districts 1 82 



(a) Staffordshire. 



(b) Nottinghamshire, 

 (r) Vale of Eden. 

 {d) The Weald. 



IV. Suggested Nomenclature : ' Pillars,' ' Balls,' and 



■ Domes ' 187 



V. ' Downward Percolation ' Theory 188 



VI. ' Volcanic ' Theory 190 



VII. Anhydrite in its Relation to Gypsum 191 



(a) Field Evidence. 



(b) Microscopic Evidence. 



VIII. Fibrous Gypsum 199 



IX. Conclusions 200 



X. Bibliography 201 



I. Introduction. 



It is now generally accepted that the extensive beds of gypsum 

 that occur in this country are the deposits of salt lakes or inland 

 seas, and the opinion has been steadily growing that the sediments 

 with which they are usually associated are the products of 

 weathering of an arid region. 



In recent years the geographical conditions of the periods when 

 gypsum was deposited have been discussed by numerous writers, 

 including Beasley, Bosworth, Burns, (leikie, Goodchild, Hewitt, 

 Jukes-Browne, Lomas, Metcalfe, Wade, and Watts. 



The object of this communication is not so much to discuss the 

 geography of these periods, as to throw some light upon the actual 

 mode of accumulation of gypsum, chiefty from the standpoint of 

 the field-observer. 



In any question dealing with gypsum, however, it is almost im- 

 possible to avoid dealing also with the anhydrous form of calcium 

 sulphate, since the two minerals occur so frequently in intricate 

 and puzzling association. The main point at issue is whether 

 gypsum is to be regarded as an original deposit, or as a secondary 

 formation. 



We may dismiss the isolated occurrences of gypsum which result 

 from the decomposition of sulphides in rocks containing lime, as 



