62 



S. L. Penfield — Crystal Drawing. 



37 



For the normal group of the isometric system, the forms 

 observed on a specimen of magnetite in the Brush Collection, 

 from Achmatowsk, Ural Mts., figure 34, has been chosen. The 

 figure was drawn by Mr. B. G. Van Name when a student in 

 the writer's laboratory. The combination is unusually com- 

 plex, trapezohedron m (311) and two hexoctahedrons, v (531) 

 and w (21 . 7 . 5), besides the simple forms a, o and d. A sim- 

 ilar combination, but with somewhat different development of 

 the forms, is described by Kokscharow.* In the construction 

 of the complex clinographic figure, the 

 orthographic projection proved to be a 

 great help. 



Both in drawing and in the study of 

 forms of lower symmetry, orthographic 

 projections are very helpful. Figure 35 

 represents the diploid s, (321), and figure 

 36 a combination of cube a and tetra- 

 hedron o. It is the writer's experience 

 that the average student has great diffi- 

 culty in gaining an idea of tetrahedral 

 forms from figures in clinographic pro- 

 jection, yet a combination of cube and 

 tetrahedron if orientated and looked at 

 from above, in the direction of the verti- 

 cal axis, will appear exactly like the 

 orthographic projection of figure 36, 

 hence the value of the figure. 



Figure 37 represents a simple combi- 

 nation of the tetragonal system observed 

 on apophyllite ; prism of the second order a, base c, and 

 pyramid of the first order p (111). The clinographic projec- 

 tion alone gives a very satisfactory idea of the general pro- 

 portion of the crystal, but the imagination must be drawn 

 on to grasp the idea that the pyramid is tetragonal, a property 

 which is brought out by a glance at the accompanying ortho- 

 graphic projection. 



Figure 38 is a combination belonging to the tri-pyramidal 

 group of the tetragonal system, observed on scapolite from 

 Templeton, Canada. The forms are two prisms a and m, 

 terminated by pyramids of the first order^ (HI) and w (331), 

 of the second order e (101) and of the third order z (311). 

 From the standpoint of a student desiring to understand the 

 relations of the three kinds of pyramids of this group, it is 

 believed that the orthographic is the most helpful of the two 

 projections, although the clinographic is needed to give an idea 

 of the general proportions of the crystal. 



* Mineralogie Kusslands, vol. iii, p. 47. 



