198 MR. B. SMITH ON THE L V(H - lxx i v » 



naturally where the attack of solutions has been broadside to the 

 anhydrite-crystals. Ophitic plates of gypsum occur sporadically 

 enclosing little fragments of anhydrite, and are somewhat similar 

 to those occurring in the body of the anhydrite away from the 

 junction, as described above. 



In both of the above cases there are also numerous rhombs of 

 dolomite, scattered through both gypsum and anhydrite, which 

 appear to have settled out at an early stage in the process of 

 crystallization of the sulphate of lime. Their average size is 

 ^ mm. in diameter, and the larger ones are from -^\ to -^ mm. in 

 diameter. 



Thus we are compelled to accept the conclusion that some of the 

 anhydrite has been converted into gypsum ] near the present line 

 of contact at least. 



In attempting to estimate the amount of the change, and the 

 time of the change, we must remember that : — 



(1) No sign of pseudomorphs can be detected a very short 

 distance from the contact, and that the larger crystals of gypsum 

 occur most frequently away from the zone of contact. 



(2) Ophitic plates of gypsum, which must be regarded as 

 original, occur in the anhydrite both near the bottom, and also 

 well away from the zone of contact. 



From the latter we infer that gypsum settled out at a late stage 

 from the mother liquor from which the anhydrite was formed. 

 There must therefore have been rather delicate chemical balances 

 capable of being easily upset by changes in temperature or con- 

 centration. 



The sequence of events seems to have been somewhat as 

 follows 2 : — 



(1) Concentration of solutions, due chiefly to evaporation through rise in 



temperature. 



(2) Deposition of lower gypsum with larger crystals of selenite, up to a 



temperature of about 36° C. 



(3) At about 36° C. anhydrite begins to crystallize out. Perhaps a little 



solution of topmost gypsum due to temporarily lessened concentra- 

 tion of calcium sulphate. 



(4) At over 36° C. anhydrite deposited in sheaves of crystals, with 



mother-liquor in the interstices. 



1 Stages in the conversion revealed by the microscope have been described 

 by F. Hammerschmidt (' Beitrage zur Kenntniss des Gyps- & Anhydrit- 

 Gesteines' Tscherm. Min. Petr. Mitth. vol. v, 1883, pp. 275-79), whose 

 conclusions in the main must be accepted. In one case isolated fragments 

 of anhydrite have regular contours, and are each surrounded by a retinue 

 of tiny fragments separated by gypsum from the margin of the central anhy- 

 drite (pi. ii, fig. 6). They become smaller outwards until they merge into 

 the gypsum. Some well-separated fragments have the same optical reaction, 

 and others are now pseudomorphs after anhydrite. Conversion of anhydrite 

 to gypsum in the United States has also been proved on microscopic examina- 

 tion by American geologists. 



2 For further evidence of seasonal changes in Keuper times, see my paper 

 on ' The Upper Keuper Sandstones of East Nottinghamshire ' Geol. Mag. 

 *dec. 5, vol. vii (1910) pp. 310-11. 



