Vol. 67.] EASTERN DESERT OF EGYPT. 257 



Xow, although I have shown that the alteration of limestone to 

 gypsum is proceeding at present in certain areas, there are many 

 reasons for doubting the conclusions of the Survey geologists. In 

 the first place, the gypseous series is of enormous thickness : reaching 

 several hundreds of feet in the Zeit range, of almost pure white 

 saccharoidal gypsum with beds of pale-blue anhydrite, and a variety 

 which is pure white. The beds are in places both overlain and 

 underlain by unaltered limestones, a fact which appears to be fatal 

 both to the Survey theory and to Mitchell's older theory. The 

 best evidence comes from recent borings. 



Borings for oil throughout this region pass through a great series 

 of beds consisting of gypsum, rock-salt, thin dolomitic limestones, 

 and marls. At Jemsa and on Jubal Island the gypsum deposits are 

 of great thickness, and the dolomitic beds are well developed ; but 

 the rock-salt is not so abundant. Borings to the north of these 

 localities meet with converse conditions. The rock-salt with 

 marly partings is most abundant, while the gypsum and dolomitic 

 limestone are more rarely present as partings. It can be shown 

 that there is some transition from one state of things to the 

 other. 



If we take the Ashrafi reef as centre and the southern ex- 

 tremity of the Jemsa peninsula as radius, and describe a circle, all 

 borings near the centre of the circle pass through the rock-salt 

 phase, while the borings on the rim of the circle pass through the 

 gypsum-dolomite phase. At one or two horizons beach- deposits 

 occur, with some sand and pebbly gravel : these suggest temporary 

 encroachments of terrestrial conditions. This association and this 

 distribution of beds seem to prove almost beyond doubt that the 

 great gypseous series is an original deposit. The finding of fish- 

 scales in the marly beds at the base of the gypsum series by Mr. J. 

 Wells, and perfect fish-remains in marly partings higher in the 

 series by Mr. C. Sara, sets the matter, however, beyond question. 



The area in which the beds are found was evidently a land- 

 locked basin that had no commucication with the sea. The position 

 of the salt-deposits with regard to the main gypsum-deposits is just 

 what one would expect from a theoretical consideration of such 

 conditions, and would not be likely to occur if the beds had originated 

 by subsequent alteration. 



Similar occurrences of salt, gypsum, and dolomite occur in many 

 parts of America, chiefly in Texas and California. They have been 

 fully dealt with by the geologists of the United States Survey. 1 

 Some of the gypsum of the Zeit range and elsewhere along the 

 coast appears to be, in places, a secondary product derived from the 

 principal beds by the action of rain and storm-waters, and re-formed 

 on the plains. The wadis cutting through the gypseous series are 

 invariably narrow and tortuous gorges with perpendicular walls. 

 Where they debouch upon the plains there is a soft, white, ashy deposit, 



1 Bull. U.S. Geol. Surv. Nos. 213, 223, 225, 260, 285, 315, 364, etc. 



s 2 



