152 



Ford and Tillotson — Orthoclase Twins. 



Arm ell. Creek occurrence, supposing that the two magmas were 

 alike, it would all be accounted for by the orthoclase itself. 

 In the present analysis the somewhat disproportionately low 

 percentage of silica is probably due to the small amount of 

 kaolinization which the feldspar showed in the thin sections. 

 Drawing of the Crystals. — The crystals were drawn from 

 a stereographic projection of their forms. There was nothing 

 new in the methods employed, but as concrete examples of the 





use of the stereographic projection for the solution of such 

 problems are seldom to be met with in the literature, a brief 

 description of the manner of transposition of the poles of the 

 faces from normal to twin position is here given.* According 

 to the Baveno law of twinning the n (021) face becomes the 

 twinning plane and as the angle c^n — 44° 56 1/2' the angle 

 between c and c' (twin position) becomes 89° 53'. For the 



* For a general discussion of the graphical use of the stereographic projec- 

 tion, see Penfield, this Journal, xi, 1, 1901. 



