﻿10 



PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [Nov. 6, 



or smoky tint, arising, as in the quartz of protogine, from the pre- 

 sence of a small quantity of organic matter. 



The orthose is of a fine rose-colour, red, or pale red, and reminds 

 one of that of the orthose of the syenite of the Vosges, which, how- 

 ever, is of a much livelier colour ; it is in crystals 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 

 calcination 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 takes a brownish 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 fatty 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 ob- 

 served in some specimens from Syene, and in which it is very abun- 

 dant, even more abundant 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 amphibolous 

 granites, a little oxidulated iron. 



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

 nished 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 differ- 

 ent 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 present, and par- 

 ticularly less mica than might have been believed on inspection. 

 This optical illusion is very general, and is to be ascribed to the cir- 

 cumstance that minerals which have lively and bright colours like 

 orthose, and particularly mica, strike the eye much more forcibly 

 than quartz, which has a grey and somewhat tarnished colour. 



I have also analysed the syenite of Egypt, of which I pulverized 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 amphibole was observed in it ; I found it to con- 

 tain : — 



