﻿1850.] DELESSE ON THE ROSE-COLOURED SYENITE OF EGYPT. 11 



Silex 



Alumina 



Oxide of iron with manganese .... 



Lime 



Alkalies and magnesia (din .) 

 Loss by fire . , 



70-25 

 16-00 

 2-50 

 1-G0 

 9-00 

 0-65 



100-00 



When the composition of this Egyptian syenite is compared with 

 that of the syenite of the Ballons of the Vosges, it is found to ap- 

 proach that of the latter*. Its proportion of silex, which is 70 per cent., 

 is indeed the same. I have already had occasion to observe, that a 

 syenite always containing hornblende, like that of the Ballons, may 

 afford upwards of 30 per cent, of quartz, and that its mean richness 

 in silex may be equal to that of many granites : this shows then, that 

 quartz is not always, as certain geologists seem to believe, merely an 

 accessory and unimportant element of certain syenites well-character- 

 ized, like those of the Ballons. As to the proportion of alumina in 

 the syenite of Egypt, it is shown by the previous experiment to be 

 tolerably great, for it is only inferior by some hundredth parts to that 

 of orthose ; and this is accounted for by the abundance of the two 

 feldspaths in the analysed specimen. 



The proportion of iron must be particularly attributed to the mica 

 and hornblende, both of which are rich in iron. 



The proportion of lime, which is sufficiently great for a granite, is 

 on the contrary very feeble for a syenite ; it is also less than that of 

 the syenite of the Ballons, which is about 3 per cent. : this results 

 from the presence of the oligoclase and of some hornblende. 



In short, the mean chemical composition of the syenite of Egypt 

 does not sensibly differ from that found for various granites ; and in- 

 deed, as I showed at the beginning, it nearly always contains much 

 quartz. It may then be regarded as an amphibolous granite, or as a 

 rock forming a passage from the family of granite to that of syenite. 



From the interesting researches of Messrs. Russegger f and New- 

 bold J on the Geology of Egypt, it results that the granite rocks occupy 

 but a very small extent ; they show themselves particularly at the 

 cataract near Syene, and in the desert where they separate the Nile 

 from the Red Sea, in the latitude of Koseir, about 26° N. 



The syenite in particular is found half a league north of Syene, 

 and according to Russegger, it extends a good deal to the south of the 

 cataract and the island of Philse into Nubia ; and it is found at Ele- 

 phantine and the intervening islands. From the collection of Lefevre 

 it appears also to have been met with in the Djebel Gareb and Djebel 

 Elzede (mountain of oil), between Koseir and Suez. 



The syenite generally disappears under a brown freestone, which 

 according to M. Russegger is again found with the same characters 

 in Upper Egypt, in Nubia, and in Sinai. This freestone appears to 



* See Annals of Mines, 4th Ser. vol. xiii. pp. 688 and 693. 



f Russegger, Reisen in Europa, Asien, und Afrika, u. s. w. Stuttgart, 8vo. 



% Loc. cit. 



