264 Professor Delesse on the 



brown pitch. This coating, so strongly united with the rock 

 as to be quite inseparable, is considered by M. Russegger to 

 be oxidulated iron.* 



Messrs Russegger and Lefevre were struck by the fact, to 

 which they frequently afterwards alluded, that the syenite 

 of Egypt is traversed by a multitude of large veins of diorite, 

 which is particularly the case along the cataract, near Philse, 

 in the neighbourhood of Syene, &c. ; these diorite veins are 

 however well known, for they also were worked by the 

 Egyptians, t 



This association of syenite and diorite is not accidental ; and 

 I have made similar observations on all the syenites that I 

 have studied in situ. Indeed I could almost always find that 

 they were associated with diorites. Thus, at the Vosges 

 especially, the syenite of the Ballons is accompanied by dio- 

 rites, which are sometimes at the bottom, sometimes on the 

 sides of the Ballons of Alsace and Conte, forming either 

 veins, fairly separated from the syenite and enclosed by it, 

 or dykes uniting with and insensibly passing into the syenite. 

 It would hence appear, that the development of hornblende 

 in the syenite is in intimate relation with the contents of the 

 veins of diorite enclosed therein, and subsequently meta- 

 morphosed into hornblende. 



It is however, necessary to add, that if the syenite be 

 generally associated with diorite, the contrary is not always 

 the case ; also, if a diorite form a vein in a granite, it must 

 not thence be concluded that crystals of hornblende had been 

 therein developed by that circumstance alone, and that such 

 granite had been metamorphosed into syenite ; at the Vosges, 

 for instance, the granite is sometimes traversed by veins of 

 diorite, and yet is not hornblendic. 



The Egyptians made great use of syenite ; subsequently it 

 was worked by the Greeks, and after that by the Romans. 

 The syenite of Egypt is still sometimes employed instead of 

 marble, and its price may be approximately estimated at 200 

 francs per square metre polished ; it is brought as ballast by 



* Russegger, vol. ii., p. 321. 



t Ibid., vol. ii., pp. 320, 322, 326, &c. 



