490 J. M. Blake — Plotting Crystal Zones on Paper. 



way we could have a control by using the tangent projection 

 system which we would lose when relying on a few single 

 measured angles. 



Since the crystals of epidote came from different sources, 

 and had different isomorphous compositions, as is indicated by 

 the analyses, it would not seem impossible that there might 



Fig. 2. 



be different proportional developments of the prismatic planes 

 in this zone in the different specimens, and that disconnected 

 measurements of angles might not always be successful in 

 bringing the prism in the correct position to make a correct 

 reading of the planes to accord with the originally adopted 

 position. This is suggested as a possible result of there being 

 two well developed tangent equal-space positions, though the 

 spacings slightly differ, and one set does not include all that 

 are in the other set. In fact, this last mentioned point led to 

 the suspicion that there was some duplication. 



