MINERALOGY OF BLACK LAKE AREA. 
53 
are not simple, but each is made up of two planes, inclined to one 
another at angles varying up to hall a degree, their line of intersection 
being parallel to the prism edge. Faces of the unit pyramid are usually 
Figure 17. Vesuvianite, 
lilac crystal from the Mon- 
treal chrome pit; average 
habit when m(110) is larger 
than a(lOO). 
treal chrome pit; average 
habit when a(100) is larger 
than *»(110). 
striated parallel to their longer diagonal, i.e. f in the direction of the apex 
of the crystal, as indicated in Figure 19. The paler crystals are typically 
freer from striae than are those of a deeper lilac colour; these nearly 
