24 
Tlic A)}icricaii Geologist. 
January, 1898 
100 
or in oio. The angle in the brachypinacoid which the Hne of 
extinction, for example, makes with the edge is called posi- 
tive if the direction of the line is included in the obtuse angle 
Fig. 19.— Optical signs of the extinction angles according to Schuster's rule. 
formed by the edges ooi : oio, oio : lOO, and negative if its 
direction lie in the acute angle of these same edges. The same 
rule is applied for a line lying in the base ooi. Figure 19 rep- 
1 I + 
Fig. 20.-Plane 010. Fig. 21.-P]ane 001. Fig. 22.- Plane 001. 
Illustrations of the sign of the extinction angle according to Schuster's rule. 
resents a simple plagioclase crystal with lines drawn in the 
faces ooi and oio, with their directions expressed according to 
Schuster. Figs. 20, 21 and 22 represent faces and sections that 
may arise in the application of this rule. 
The principal element is the optic plane. In it lie the optic 
axes and the bisectrices. Its position in the crystal has definite 
relations to the cleavages, the external faces and the crystal- 
lographic axes in the different feldspars, and its angle with the 
albite twinning, in the plagioclases, is the key which furnishes 
