0. Andersen — Aventurine Feldspar. 355 



Planes of Orientation. 



As the specimens consisted of cleavage pieces, in general 

 showing no other faces than the cleavage faces (001) and (010), 

 all measurements had to be referred to the elements (001) 

 (010) and the a-axis. 



In determining the planes of orientation of the lamellae we 

 have thus to deal with the following angle coordinates : 



p P = angle between P (001) and lamellae. 

 p M = angle between M (010) and lamellae. 

 <f> P = angle between line of intersection : 



lamellae A P (001) and a-axis. 

 (£ M = angle between line of intersection : 



lamellae aM (010) and a-axis. 



A combination of two of these angles determines the plane 

 of orientation of a set of lamellae. 



To be able to refer the measurements to the proper octants 

 in the axial system of the feldspar we must know the approxi- 

 mate direction of the c-axis (or the direction of the positive or 

 negative a-axis) for each cleavage piece examined. In the 

 plagioclases the difference between the obtuse and the acute 

 edges of the a-axis must also be noticed. On specimens of 

 microcline the perthite striation on (010) will generally indi- 

 cate the approximate direction of the c-axis. In the plagio- 

 clases it is necessary to look for indications of the third cleav- 

 age, parallel to (110), or else to rely on the determination of 

 extinction directions on small cleavage pieces chipped off from 

 the larger. 



The angles <j> could be measured either with the microscope 

 on oriented sections after (001) and (010), or with the goni- 

 ometer on cleavage pieces. The angles p were always measured 

 with the goniometer. 



Measurements with the microscope. 



The measurements of the angles cj> on thin sections consisted 

 in determining the angle between the cleavage lines and the 

 lines of intersection of the lamellae with the surface of the sec- 

 tion. The latter lines we will call the section lines of the 

 lamellae. 



As there were always more than one set of lamellae to be 

 measured, the difficulty in determining the orientation of each 

 set by measuring the two angles <p would consist in a correct 

 correlation of the measurements from the two different sec- 

 tions. For each set, the angle (f> of which had been determined 

 in one section, it was necessary to estimate the angle p in the 

 same section in order to get an idea of the orientation of the 



