B. S. Butler — Pyrogenetic Epidote. 31 



indicates that the quartz and epidote were among the earliest 

 minerals to crystallize ; both were earlier than the feldspar 

 and biotite, at any rate the latter minerals do not show the 

 corrosive effects that characterize the former. 



The epidote possesses the optical properties characteristic of 

 that mineral. Pleochroism a pale greenish-yellow, b pale 

 lemon-yellow, c nearly colorless. Absorption c>6> a. Twin- 

 ning- plane 100. Cleavage 001 and 100 distinct. Plane of 

 optic axes 010. a^<s— 2° 25' average of several readings. 

 Angle between 100 and 001 = 114° 26' average of measure- 

 ment on several crystals. Optical character (— ). y — a= '024, 

 determined by table of birefringences. 



A separation of the epidote was made by breaking the 

 crystals from the matrix. In this manner material that was 

 fully 50 per cent epidote was obtained. This was crushed to 

 pass a 100-mesh sieve and the powder separated by Thoulet's 

 solution at maximum density. The material obtained was 

 examined microscopically and found to be practically pure 

 epidote. An analysis of this material by Mr. W. T. Schaller, 

 of the U. S. Geological Survey, gave the following composition : 



Analysis of Epidote from Shasta Co., Cal. 



Si0 o 38-22 



Ti0 2 _ 0-33 



A1„0„.__. 25-12 



Fe" o 0° 3 8-75 



FeO 1-25 



MnO 0-19 



Mo'O trace 



CaO 22-77 



K„0 -06 



Na a O _ -11 



H 2 0- -52 



H 2 0-f .... 3-04 



100-36 



Rare earths none 



Density (approximately) 3*29 



It is seen that, disregarding the minor constituents, the 

 mineral conforms very closely to the formula (Ca,Fe) 2 (AlOII) 

 (Al, Fe) 2 (Si0 4 ) 3 , the molecular ratio of calcium to ferrous 

 iron being CaO : FeO : : 24 : 1, and that of aluminium to ferric 

 iron Al 2 3 :Fe 2 3 :: 4-5:1. 



Aside from the foregoing evidence of the primary origin of 

 the epidote there are additional reasons for believing that it is 

 not secondary. The large crystals of epidote appear with 



