256 ME. A. AVADE ON THE [May I9II,. 



Calcium chloride was proved to be present in the limestone bearing 

 the encrustations, and these were accompanied by the presence 

 of fine transparent needles of very small size. The needles in- 

 creased in quantity after specimens had been kept for a time, and 

 were evidently a kind of efflorescence. They have low refractive 

 index and bi-refringence, and give straight extinction. Analysis 

 proved them to be the mineral epsomite (MgS0 4 7H.,0). 



(!>) Dealing now with the second type of gypsum and salt deposit, 

 we are at once confronted with an attractive but somewhat difficult 

 problem. An enormous series of gypsum and anhydrite deposits is 

 present in the area which we are considering. They form beds 

 hundreds of feet thick on the Jemsa peninsula, in the Zeit range, 

 and elsewhere, localities not previously noted being the centre of 

 Jubal Island and the northern end of iShadwan Island (see raap r 

 fig. 4, p. 255). 



The beds of anhydrite are sometimes peculiar. The mineral 

 occurs as a soft chalky-looking substance with a radiate structure. 

 It looks as though the mineral had been formed from a bed of 

 gypsum in which the crystals were present in radiating aggregates, 

 resembling a spherulitic structure on a fairly large scale. An 

 analysis of this anhydrite gave the following result : — 



Per cent. 



CaO 38-70 



MgO 208 



SO,, 5930 



Na!,0 and K 2 trace 



H.p 012 



Total 100-20 



Barron & Hume 1 consider that these deposits represent ' gyp- 

 sumized ' limestones of the Eocene and Cretaceous formations. They 

 are of opinion that the change has taken place from above, by the 

 introduction of sulphuretted hydrogen due to the decay of organic 

 matter in the raised-beach deposits with which these beds are 

 often associated. 



L. H. Mitchell, 2 on the other hand, connected the change wilh 

 the igneous intrusions ; and the close connexion between the beds 

 of gypsum and the veins of sulphur formerly worked at Has Jemsa 

 would support his opinion. Moreover, hot water has been struck 

 in two or three localities in course of the search for oil : this also 

 could perhaps have played a part. Mitchell was of opinion that the 

 granites of Jebel Zeit were intrusive into the overlying limestone 

 series, but this is not confirmed by later studies of the locality. 



1 Op. cit. Egypt. Geol. Surv. Eep. 1902, pp. 192-97. 



2 ' Eas Gemsali & Gebel Zeit: Report on their Geology & Petroleum 

 [Egypt. Govt. Eep.] Cairo, 1887. 



