112 



ROGERS— THE VALIDITY OF 



[March i, 



stituting various styles of crystal architecture, are necessary to 

 account for the crystals of various systems. 



The crystal faces of most frequent occurrence are, according to 

 Bravais, those planes in which the points of the space-lattice are 

 most closely packed. These are faces with simple indices as can be 

 seen from Fig. 3. Faces with complex or high indices are planes 

 with the points relatively far apart (Fig. 3 ). It is a well-known fact 



ICO 



Fig. 3. The relative frequency of occurrence of crystal faces. 



that the same forms are not common for every crystal even of the 

 same system. The space-lattice is distinctive for every crystal ex- 

 cept those of the isometric system and even in this system there are 

 three kinds of space-lattice possible. The following statistics, com- 

 piled from Goldschidt's " Krystallographische Winkeltabellen," will 

 give an idea of the relative abundance of the various forms. Tak- 

 ing the (hko) zone for 206 orthorhombic minerals, thus eliminating 

 individual peculiarities, (010) occurs on 134, (100) on iii, ( iioj on 

 144, (120) on 66, (210) on 48, (130) on 43, (230) on 29, (320) on 

 22, (310) on 20, (150) on 17, (430) on 10, (340), (530), and (410) 

 on 9 each, (540) on 8, (610) on 7, (560) and (350) on 6 each, 

 (160), (250), (520), and (To-9-o) on 5 each (650) and (750) on 4 

 each, (170), (710), and (740) on 3 each, (510) (970), (i-ii-o), 

 (i-i2-o),on 2 each, and many forms including (7-ii-o), (io-7-o), 

 (i-i2-o), (i6-i-o), (i9-20-o), etc., on one each. 



