part 3] CHELLASTOX GYPSUM BHECCIA. 175 



being a minor point well understood, and confine ourselves to the 

 more extensive deposits occurring as beds and smaller masses, or as 

 a matrix to the rocks in which the thicker deposits are found. 



Various suggestions have been advanced to explain the origin of 

 these deposits : — 



(i) Some may be due to evaporation by concentration of river- water 



charged with salts, flowing into inland basins without outlet. 

 (ii) Others may be formed when an arm of the sea is isolated and 



exposed to evaporation, 

 (iii) Certain deposits have been claimed to be the results of the action 



of sulphureous emanations from volcanic vents upon limestone 



or other calcareous rocks. 

 (iv) Under certain conditions gypsum may be formed from anhydrite ; 



but, if so, we still have to determine how the anhydrite was 



deposited. 



The early part of tin's paper deals chiefly with the stratigraphical 

 aspects of the question. 1 propose, so far as possible, to postpone to 

 a later stage ' pp. 195, 198) any theories upon the possible chemical 

 changes and balances (which have been discussed by many authors *) 

 necessary to produce precipitation either of gypsum or of anhydrite, 

 and. my final endeavour will be to show that many of the 

 gypsum-anhydrite deposits in this country must have 

 been formed ab initio in stratiform manner, and are 

 not the result of subsequent changes, additions, or 

 replacements of any importance. 



Deposition in Inland Basins. 



The majority of the British deposits were apparently formed as 

 chemical precipitates on the "Moors of inland basins cut off from the 

 open sea, but fed by rivers (in the case of the Permian and Trias, 

 at least) bearing sediment derived from a country of low average 

 rainfall and high average temperature. The deposits may be classed 

 as those of salt lakes, as distinct from bitter lakes. 



In some eases the salt lakes may have been portions of the ocean, 

 isolated and fed by rivers; while in other cases they may have 

 been inland lakes formed solely by river- waters, which may, 

 however, have dissolved — from the rocks within their drainage- 

 areas — oceanic salts that were deposited in earlier geological ages. 



If to these conditions we add change in temperature, also earth- 

 movements which would at times cause one basin to open into 

 another or cause diversion in river-drainage, and alteration in 

 climatic conditions, we have all the circumstances necessary for 

 the precipitation of mineral salts contemporaneously with the 

 peculiar sediments of such areas. We should not expect to find 

 an unbroken sequence of saline deposits, such as would result from 

 the steady and uninterrupted evaporation of a given amount of 



1 Sir Archibald Geikie, ' Text-Book of Geology' 4th ed. vol. i (1903) pp. 525 - 

 31 ; F. W. Clarke, ' Data of Geochemistry ' 3rd ed. Bull. U.S. Geol. Surv. 

 No! 616 (1916) pp. 226-28 ; J. L. A. Both, ' Allgemeine & Chemische Geologie ' 

 vol. i (1879) pp. -163 et seqq. 



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