Blake — Solving Crystal Problems. 



655 



arated projection planes. This development of like fig- 

 ures cannot be a coincidence simply, but must have a 

 meaning. A further study of these orthoclase projec- 

 tions will show other minor resemblances or coincidences 

 if we take into account a difference of scale. There is, 

 therefore, much connected with the relations among the 

 planes in this mineral species that deserves investiga- 

 tion. 



Albite is a tricinic species of soda feldspar. It 

 yields some features of interest when tested by the pro- 

 jection method. Six projections of albite have already 

 been given in article 4, and to save space only two of 

 those recently projected will be selected for our present 

 purpose. The angles calculated by Des Cloiseaux and 

 his stereographic projection were made the starting 



Fig. 4. 



\t 77^ 



*>%• 



*7. 



Fig. 4. Albite. 



point for the present trial of the method. Seven projec- 

 tions were made in this recent trial, and they were found 

 to give the characteristic equal spacings. 



One of these projection trials is shown in fig. 4. It 

 has the same basal zone as that used by Des Cloiseaux 

 in his stereographic projection. It affords an insight 

 into his system of assigning symbols, and a point of 

 interest is that it gives a very compact assemblage of 

 the planes. A model of albite had been cut with all of 

 the planes tangent to a sphere, and therefore, this model 

 gave an unbiased development to all the planes. When 

 used as a test, the model showed a prism development 

 in favor of the prism selected by Des Cloiseaux on 

 which he based his stereographic projection and his 

 indices for the planes. The figure 141 in his Mineral- 

 ogy, however, showed a decided prism elongation in 

 another direction, and this latter prism has been used as 



