150 



Ford and Tillotson — Orthoclase Twins. 



one, not so completely formed, is merely grown on to the side 

 of the larger. The bottom plane of this twin is made up of the 

 c (001) face of the first and the b ! (010) face of the second individ- 

 ual, the division between the two being clearly recognizable 

 because in these crystals the clinopinacoid has always a somewhat 

 brighter luster than the base. In this case the dividing line 



between the two faces is almost straight. In the crystal repre- 

 sented in figure 3, the two individuals are more nearly equally 

 developed, and here again the division between the two seems to 

 be almost in the nature of a single plane. The composite face 

 of this crystal is the one in back and to the left as the crystal is 

 drawn and the distinction between the c and b faces is clearly 



