262 Professor Delesse on the 



I have also analysed the syenite of Egypt, of which I pul- 

 verized a large piece, obtained from the Egyptian Museum 

 in the Louvre, and placed at my disposal by M. Dubois, one 

 of the conservators. It presented the general characters 

 which have just been described, only that some hornblende 

 was observed in it; I found it to contain: — Silica, 70*25 — 

 Alumina, 16'00 — Oxide of iron with manganese, 2*50 — 

 Lime, 160 — Alkalies and magnesia (difF.)j 900— Loss by 

 fire, 0-65 = 10000. 



When the composition of this Egyptian syenite is com- 

 pared with that of the syenite of the Ballon s of the Vosges, 

 it is found to approach that of the latter.* Its proportion of 

 silica, which is 70 per cent., is indeed the same. I have al- 

 ready had occasion to observe, that a syenite always contain- 

 ing hornblende, like that of the Ballons, may afford upwards 

 of 30 per cent, of quartz, and that its mean richness in silica 

 may be equal to that of many granites ; this shows then, 

 that quartz is not always, as certain geologists seem to be- 

 lieve, merely an accessory and unimportant element of cer- 

 tain syenites well-characterised, like those of the Ballons. 

 As to the proportion of alumina in the syenite of Egypt, it is 

 shewn 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 feld- 

 spaths 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 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 indeed, as I shewed at the beginning, it gene- 

 rally contains much quartz. It may then be regarded as 



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



