Rose- Coloured Syenite of Egypt. 26t 



which, however, is of a much livelier tint ; it is in crys- 

 tals of some centimetres in size, which are mackled like 

 the orthose of granite rocks ; this is the most obvious, and 

 frequently the predominating mineral of the rock, giving to 

 it its general reddish hue. Its density is 2*568. By calcina- 

 tion it loses only 0*35 ; this loss is very slight, and such as 

 generally takes place with orthose. 



In a state of decomposition it sometimes becomes of a 

 brouTiish colour, owing to the release of the small portion 

 of oxide of manganese held in combination. 



The feldspath of the sixth system has not the greasy lustre 

 of that of the syenite of the Vosges, and it seems to me it 

 ought to be regarded as oligoclase ; it is most frequently 

 white ; sometimes however, it becomes yellowish, or even 

 greenish, as is for instance observed in some specimens from 

 Syene, and in which it is very abundant, even more abund- 

 ant than the orthose. 



The mica, rich in magnesia and iron, occurs in bright 

 spangles, often black, but according to De Roziere, sometimes 

 brown or green. When black, their colour resembles that 

 of the hornblende, which is often associated with the mica. 



There is often present iron pyrites, and, as in all hornblendic 

 granites, a little oxidulated iron. 



Occasionally also, but very seldom, garnet occurs ; it is of 

 a tarnished brown colour, and crystallized in the form of the 

 rhomboidal dodecahedron. 



I have determined, by the process described in the Annals 

 of Mines (4th S., vol. xiii., p. 379), the proportions in bulk 

 of the different minerals contained in a polished specimen of 

 the rock, and have obtained : — Red orthose, 43 — Grey quartz, 

 44 — White oligoclase, 9 — Black mica, 4. 



This specimen, which was very rich in quartz, did not seem 

 to me to contain hornblende ; there was also less orthose pre- 

 sent, and particularly less mica than might have been be- 

 lieved on inspection. This optical illusion is very general, 

 and is to be ascribed to the circumstance that minerals 

 which have lively and bright colours like orthose, and parti- 

 cularly mica, strike the eye much more forcibly than quartz, 

 which has a grey and dull colour. 



