Johns : The Permian Salt Lake. i77 



evidence required for our investigation is all that one could 

 ask for. 



Gypsum is of course sulphate of lime, with its water of 

 crystallisation, and may be represented by its formula, 

 CaSO^ 2H2O. Anhydrite is sulphate of lime without the water. 

 Now gypsum is a very ordinary deposit in a salt lake which has 

 reached a certain degree of concentration, but the formation of 

 anhydrite only takes place under certain well defined conditions. 

 These conditions were determined during the investigations* of 

 Vant' Hoff and his pupils on the famous Stassfurt salt deposits. 

 It will now become clear why stress has been laid on the presence 

 of gypsum beds both above and below the anhydrite, and separ- 

 ated from it by marl. It is the fact that they appear in these 

 positions that supplies the proof that the conversion of the 

 gypsum into anhydrite was contemporaneous, and that the 

 particular conditions attending its formation must have been 

 peculiar to upper marl time. 



Vant' HoflF's experiments t prove that gypsum is converted 

 into anhydrite only when the temperature is about 3o°C. and in the 

 presence of a saturated solution of sodium chloride, common salt. 

 The conversion is a slow one, and though it might have a slight 

 range above and below, it affords us a reliable figure for deter- 

 mining the temperature of the lake for a considerable period. 

 The lake was shallow, and the eflfect of pressure on the trans- 

 formation temperature would be so small as to be negligible. 

 We are thus presented w^th interesting data of an unexpected 

 character, and a discussion of the climatic conditions at that 

 remote period has in this particular instance the advantage of 

 being based on something better than doubtful lithological 

 features or obscure paleontological evidence. 



The simple character of the salts in the Haxey bore, viz.^ 

 gypsum and anhydrite, differentiate these deposits from those 

 composing the Stassfurt beds, where there is ample evidence 

 that the sea had access at times to the basin in which the 

 complex series of salts found there were being deposited. In 

 our shrunken Midland basin the lake was landlocked, and there 

 is no evidence of occasional inroads of the sea. The marls 

 testify to rushes of fresh water bringing down sediments from 

 the surrounding land surface, and the gypsum is evidence that 



* A resume of \';int' HofTs work by Professor Armstrong; appeared 

 in ' Rep. Brit. Assoc.,' p. 262, et sequa. References occur in various Nos. 

 of ' Science Abstracts ' during- recent years. 



t 'Archives Nt^erlandaises ' 6, pp. 471-489, 1901. Reference in 'Science 

 Abstracts' 1902, p. 219. 



1906 June I. 



