656 



Blake — Solving Crystal Problems. 



a base for the projection shown in fig 5. This last selec- 

 tion of prism gives the most symmetrical assemblage of 

 the planes of any of the albite projections, and would 

 have yielded a set of symbols nearly free from fractions 

 by the axial system as nsed by Des Cloiseaux. It will 

 be noticed that the projection fig. 5 covers the largest 

 area. The contrast is shown in figures 4 and 5. 



To obtain the projection shown in fig. 5 rotate the 

 upper front of the crystal downwards 116 degrees. 

 This brings the hidden plane p to the front side. 



Anoethite is a lime feldspar, and is a typical tri- 

 clinic species. It gives an interesting series of projec- 

 tions. Fig. 7 shows nine of these projections, all start- 

 ing from the plane g' and fig 8 gives six of the cross 

 zones, making in all fifteen. The possibilities of addi- 

 tional zones have not been exhausted, but they would 



Fig. 5. 



c'/x\ la' /h'/x 



• • 

 C b' 



7 ' * ' 



7 



»5 





Fig. 6. 





^ 



\ 



V # l K # / m 



/ 

 ^ 





? ?/♦ <& c\ 8 





1% aA 







• /ft© *\ 

 7,1 ( " ®*\ c1k ) 



? 



ft 



c?t\ 



< 



• • 



O'lx 



> A \H' t\ 



* 



Fig. 5. Albite. 



Fig. 6. Anorthite. 



take up more space, and those given will answer our 

 present purpose. The members of this series of plots 

 can be referred to by numbers leading from left to 

 right, and thus following down each of the several 

 columns. The zones are read off from left to right 

 from the front side of the crystal. 



The gnomonic projection fig. 6 is made from the same 

 position of the crystal that Des Cloiseaux selected for 

 his stereographic projection. It will assist in picking 

 out the planes by their position in the zones in the fur- 



