M. A. Delesse's Researches on Granite. 



207 



velopment is between Sainte Marie aux Mines and Guebwiller ; 

 it contains quartz, orthose, feldspar of the sixth system, dark 

 mica, and sometimes hornblende. 



The quartz is hyaline, and of a gray colour ; it is most abun- 

 dant in the highly crystalline varieties; those varieties which 

 are porphyritic and least crystalline contain little or no quartz, 

 the greater part of the silica having remained in combination 

 with a feldspathic paste. 



The orthose is the preponderating mineral of this granite. It 

 is white or reddish-yellow ; both kinds, containing oxide of iron, 

 turn red by alteration ; it sometimes becomes greenish, and by 

 decomposition passes into a halloysite. The orthose is the most 

 persistent mineral of this granite ; its crystals sometimes attain 

 a decimeter in length : the analysis of three specimens from dif- 

 ferent localities gave the following result : — 



The proportions given in this table differ but slightly from 

 each other or from previous determinations ; orthose is then a 

 mineral whose composition is very constant, and independent 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 trans- 

 lucent, and has a greasy lustre ; its crystals show parallel striae, 

 which characterize the isomorphous feldspars of the sixth system ; 

 it becomes red by atmospheric alteration, afterwards white, and 

 the mineral passes into kaoline. The analysis of it gave the 

 following composition : — 



Silica .... 

 Alumina . . . 

 Oxide of iron . . 

 Oxide of manganese 



Lime 



Magnesia . . . 



Soda 



Potash . . . . 

 Loss by burning . 



58-55 

 25-26 

 0-30 

 trace 

 5-03 

 1-30 

 6-44 

 1-50 

 0-91 



Oxygen. 

 30-422 



Ratio. 

 8 



11-807 

 0-092 



1-412 

 0-517 

 1-648 

 0-255 



}" 



899 



■3-832 



Sum 99-29 



It contains less of silica and of alkalies, with more of lime, than 

 oligoclase ; moreover, its atomic proportions of oxygen are very 

 nearly that of andesite. This strengthens a remark I have made 



