PYKOXENE-ANDESITE. 



351 



(rt) ; tomuieuting at the normal, X, with a valno of about 30°, the direction to the left 

 c<>rres])onds to that which i)assesover the obtuse angle of that half («); the light line 

 is the curve for the second half (h), its corresixmdiug normal, X', being 52° 18' to tlic 

 right of the former, and starting at this point with the same value, 30°, but with oi)p(i 

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



Via. 7 Ciirlsbaci twin orlalniiilorili-. 



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 Iti the two 

 halves are equal and opposite, that is, symmetrical with respect to the brachy])inacoid ; 

 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 



X' Plane a.1 rCght angles to loP. adPdbl ofiaJ 

 y Ptanr containing toP.ooPdbl ol" la>, 

 n Plane parallel to oerlical axis 



.X'J'lane at rigHt angles to loP.ooPobJoribl 



ypiane containing loPooP<a>lanb) 



m Plane at right angles lu vertical, axtx 



Fig. 8. — Diagram of extinctiou angles. 



sign, wliich 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 the.se 

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

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

 and the trace of the brachypiuacoid in sections in the zone iji question. The figures 



