341 



Researches on Granite. By A. Delesse.* 



[From the special study of the granitic rocks of the Vosges 

 Mountains, the author has made some generalizations of 

 great interest upon the relation of the proportion of silex, 

 and of the nature of the mica, to the age of the mass and to 

 its circumstances of crystallization, also upon the varieties of 

 feldspar.] 



There are in the Vosges at least two types of granite, dis- 

 tinguishable by their mineral ogical constitution and geologi- 

 cal position. 



The first is the granite of the Ballons ; it forms the sum- 

 mits and the central part of the ridge of the Vosges ; its 

 greatest development 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 

 abundant 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 feld spathic 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 green- 

 ish, and by decomposition passes into a halloysite. The or- 

 those is the most persistent mineral of this granite ; its crys- 

 tals sometimes attain a decimeter in length : the analysis 

 of three specimens from different localities gave the follow- 

 ing result : — 





SiO 3 . 



A1 2 3 . 



Fe 2 3 . 



CaO. 



MgO. 



NaO. 



KO. 



HO. 



Sum. 



I. 

 II. 



III. 



64-91 

 64-66 

 64-00 



19-16 

 19-58 



traces 

 traces 



0-78 

 0-70 

 068 



0-65 



2-49 



11-07 



0-30 

 0-58 



1-28 



= 9636 

 =10000 

 =100.00 



15*18 



20 



'55 





13-49 





The proportions given in this table differ but slightly from 

 each other or from previous determinations ; orthose is then 



* From the Annates des Mines, vol. iii. p. 369. 



