208 



M. A. Delesse's Researches on Granite. 



before, that all the feldspars of the sixth system are isomorphous, 

 and that their proportions of silica may vary indefinitely between 

 that of albite and that of anorthite. This feldspar of the sixth 

 system occurs in the most crystalline granite, and appears also 

 to be especially associated with hornblende. 



The granite of the Ballons contains but one mica, of a dark 

 colour, with sometimes a greenish shade. In the polariscope of 

 Amici it shows two optic axes, forming a very small angle. Its 

 dominant bases are magnesia and iron : it is affected by hydro- 

 chloric and sulphuric acids. 



The accidental minerals of this granite are hornblende, sphene, 

 zircon. 



It is very little broken or veined. The mean composition of 

 some of its varieties are — 



The dots indicate that the quantitative determination was not made. 



The loss of silica is replaced by alumina and lime. These 



variations depend very much (as I have proved elsewhere, Bull. 



de la Soc- Geol. 2nd ser. vol. ix. p. 464) upon the position in 



the mass, the more central and elevated being the more siliceous, 



and upon the nature of the rocks in junction. 



The second type of granite is the granite of the Vosges. I 



group under this name the varieties which have been called 



common granite, leptynite and gneiss. 



Its essential minerals are quartz, orthose, feldspar of the sixth 



system, two micas — a dark and a bright. 



The quartz is in grayish- white grains. The orthose is the 



preponderating mineral ; it occurs in minute lamellae or grains, 



the analysis of which gave — 



Silica 66-08 



Alumina and traces of peroxide of iron . 18*70 



Oxide of manganese trace 



Lime 093 



Magnesia 0*45 



Soda 3-77 



Potash 911 



Sum 99-04 



