Blake — Solving Crystal Problems. 657 



ther reduced plots. These several prism zones were 

 brought in place for the corresponding projections by 

 rotating the front of the crystal downwards, thus bring- 

 ing them successively into position. 



It will be noted that this series contains a number of 

 quite symmetrical plots, and each one of these fifteen 

 plots includes or should include all the planes, and could 

 be used for a description of the crystal. Some of the 

 plane positions have been inked to make them more pro- 

 minent, and to aid in tracing out the more striking of 

 the equal spaced zone rows. Planes that take a promi- 

 nent position in one plot may take a subordinate posi- 

 tion in another, and almost invariably these subordinate 

 planes, themselves, will be found to fall into zones, and 

 these zones although possibly not prominent in the pro- 

 jection plot, yet will be found to cross at the plane posi- 

 tion selected, and these subordinate planes are conse- 

 quently subject to the zone law, however complicated 

 their symbols might become if assigned to them by the 

 axial system of notation. 



There are several of these anorthite plots that could 

 be made the basis of a new and different set of symbols 

 by the axial system. There has been no well-established 

 guide to follow in these cases and one author admits 

 that it is generally expedient to follow after the writer 

 making the first description of a species. 



If we examine the anorthite projections we find that 

 the space ratio, that is, space divided by radius, varies 

 with the direction taken by each projected zone, and it 

 varies in the individual plots. One exception was noted 

 in plots 4, 6, and 14, where the spaces measured hori- 

 zontally appear to be equaL Besides the exceptions 

 just mentioned, there are other points of interest to 

 which attention has been drawn in this paper. In the 

 previous papers, methods have been suggested by which 

 the preliminary work on crystals can be more easily 

 accomplished. By utilizing such means more ground 

 can be worked over, and interesting and important 

 features may be discovered. These points of interest, 

 when given proper consideration, will tend to promote 

 progress in crystal work. 



While reviewing the results brought out in these 

 several gnomonic projections, as they were being devel- 

 oped by the graphic methods from the plots of the 



