342 M. A. Delesse's Researches on Granite. 



a mineral whose composition is very constant, and indepen- 

 dent of that of the rock in which it is produced. 



The granite of the Ballons contains also a feldspar of the 

 sixth system ; its colour on a fresh fracture is greenish ; it 

 is translucent, and has a greasy lustre ; its crystals shew 

 parallel striae, which characterize the isomorphous feldspars 

 of the sixth system ; it becomes red by atmospheric altera- 

 tion, afterwards white, and the mineral passes into kaoline. 

 The analysis of it gave the following composition : — 







Oxygen. 



Ratio. 



Silica 



58-55 



30-422 



8 



Alumina 



25-26 



Voll} »«.. 



3 



Oxide of iron 



0-30 



Oxide of manganese 



trace 







Lime 



5-03 



1-412 V, 





Magnesia . 



1-30 



fSJ 3832 



1 



Soda 



6-44 



Potash 



1-50 



0-255 J 





Loss by burning 



0-91 







Sum 



99-29 





It contains less of sili 



ca and of alkalies, with more 



of lim 



than oligoclase ; moreover, its atomic proportions of oxygen 

 are very nearly that of andesite. This strengthens a remark 

 I have made 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 ofanorthite. 

 This feldspar of the sixth system occurs in the most crystal- 

 line 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 shews two optic axes, forming a very small angle. 

 Its dominant bases are magnesia and iron : it is affected by 

 hydrochloric 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 — 





