PYKOXENE-AXDES1TE. 



351 



() ; commencing at the normal, X, with a value of abont 30, the direction to the left 

 corresponds to that which passes over the obtuse angle of that half (a); the light line 

 is the curve for the second half (b), its corresponding normal, X', being 52 18' to the 

 right of the former, and starting at this point with the same value, 30, but with oppo- 

 site sign; its passage to the left corresponds to that over its acute angle. From the 



Flo. 7. Carlsbad twill of labradoritc. 



resulting figure it is readily seen that in the plane 26 9' to the right of the normal, 

 X, which is the section parallel to the vertical axis, the angles of extinction in the two 

 halves are equal and opposite, that is, symmetrical with respect to the brachypinacoid ; 

 and that in the plane 63 51' to the left of the same normal, X, which is the section 

 at right angles to the vertical axis, the extinction angles are equal, but have the same 



n Plant; parallel to vertical axis 



m Plane at right angles to vertical axt* 



Flo. 8. Diagram of extinction augleg. 



sign, which agrees with the conclusions previously arrived at. The greatest differ- 

 ence in the size of the angles in any one section appears to be about 20. That these 

 variations occur in a great number of nearly rectangular sections is understood ujxui 

 comparing with the diagrams the following table of angles made by the basal cleavage 

 and the trace of the brachypinacoid in sections in the zone in question. The figures 



