74 WYNNE : GEOLOGY OF THE SALT RANGE IN THE PUNJAB, 



Although the gypsum occurs in great quantity, true anhydrite has 



never been found associated with it or in the marl, 

 Semi-anliydrite. 



the nearest approach to this mineral being certain 



large nodular cores of greater weight and hardness and of a bluish white 



tint within beds of the whitish gypsum. Specimens of this rock were 



found by Dr. Warth to contain only 5 per cent, of water. 



Bischof says : Sulphate of lime at high temperatures under pressure 

 crystallises with 6*21 per cent, of water, thus forming a semi-anhydrite'^ 

 (anhydrite being of course waterless and gypsum containing 20-79f per 

 cent.) , and coming close to the Salt Kange rock. Gypsum (according to 

 Bischof also) may have three-fourths of its water driven off by long ex- 

 posure to boiling temperature, leaving the same percentage nearly as in 

 Dr. Warth^s specimen. Hence it would appear that if this be an original 

 rock, a high temperature may have existed during its formation,:|: or 

 heat may have acted upon it since its deposition. If neither be the case, 

 the situation of this semi-anhydrite may suggest the transition of the 

 rock from anhydrite to gypsum by taking up water. This variety 

 melts away or changes into finely crystalline white powder below the 

 surface of a stream highly charged with salt in Khewra gorge. 



Sometimes in thick masses of the ordinary gypsum, there occur layers 

 Dolomite layers in gyp- ^f ^hite, brittle, hard flaggy dolomite which looks 

 s^^- and burns like a limestone. At one or two places, 



notably at the southern foot of Mount Tilla,§ these dolomite layers 

 contain numerous and very perfect casts of large " hopper-shaped" crystals 

 of salt. The same kind of rock becomes massive at the western foot of 

 Mount Chambal (east) northward of Jalalpiir, also in Khewra glen, 

 where a foetid variety is associated with the peculiar eruptive rock of 



* Bischof on rock-salt works near Stassfiu-t, C. E. M. PfefEer, Halle, p. 48. 

 t 20-88, Vide Page, and 20-8, Dana. 



% The gypsum of the Spiti Valley is attributed partly to thermal springs. — Mallet, 

 Mem. Geol. Surv. Ind., Vol. V, p 159. 

 § First observed by Dr. Warth. 



( 74 ) 



