Vol. 67.] EASTERN DESERT OF EGYPT. 259 



XI. Comparisons. 



The significance of these beds, both of salt and of gypsum, will not 

 be lost upon the student of the Triassic rocks of Britain. The beds 

 in the Red-Sea area are comparable in every way with those of the 

 Keuper, although the great salt-masses in the Keuper are probably 

 not comparable in thickness with those now described. 



The coasts of the Red Sea are at present rising with what 

 appears to be unusual rapidity, geologically speaking. If continued, 

 this will entail a severance of the connexion with the Indian Ocean, 

 and it seemed to me to be useful to endeavour to calculate the 

 thickness of salt which would result from the desiccation that 

 might ensue. 



Taking the average depth of the Red Sea from the Admiralty 

 Charts to be 2400 feet (probably a low estimate), the amount 

 of salts dissolved in the sea as about 4 per cent., and their density 

 2*00, * I reckon that the thickness of such a bed of salt would be 

 50 feet. It would certainly be somewhat greater than this, because 

 of certain obvious corrections that would need to be applied. 



Now, this proves that the beds actually present could not have 

 been formed by the desiccation of a sea having the salinity of the 

 present Red Sea, unless that sea in the process of drying up shrank 

 greatly in size and concentrated its salts over a limited area. 

 Occasional additions of salt water by temporary inroads from the 

 ocean will not explain matters, since such connexion with the 

 ocean would in all probability have been accompanied by other 

 phenomena, of which there is no evidence. Incursions of sea- 

 water do seem to have occurred towards the end of the period, and 

 are suggested by the alternating deposits of salt and gypsum, which 

 appear to be somewhat late in age, and seem to occur only in the 

 central parts of the area. 



If we assume that the Keuper Sea was similar in depth to the 

 Red Sea, we reach a similar conclusion, for beds of salt 100 feet 

 thick are to be found in that formation. If, as seems more likely, 

 we assume that the Keuper Sea was shallower, it is clear that its 

 waters were vastly more saline than the waters of the present 

 ocean. Now, since we have no reason to believe that the ocean 

 was at any time much more saline than at present, it follows that 

 the Keuper salt-beds were deposited in an inland sea uncon- 

 nected with the ocean, since we know that where such seas are in 

 continuous connexion with the ocean at the present day, there is 

 little difference in the salinity. 



There is, however, one strong contrast. Whereas, as Mr. A. R. 

 Horwood has recently observed, there is a tendency for the higher 

 beds to overlap the lower in Triassic times, a tendency due to 

 general subsidence, the reverse is the case in the Red-Sea area. 

 The lowest beds here tend to have the greatest extension, owing to 

 the fact that this area is one that is rising. Hence the beds which 

 would correspond with the Keuper Series occupy the smallest area, 

 occurring in hollows in the older rocks. 



1 J. H. Howell, in J. B. Skertchley's ' Physical Geography ' (31st ed.) 1896. 



