64 



S. L. Penfield — Crystal Drawing. 



orthographic projection above, accompanied by the statement 

 that the rhombohedron is very fiat, c^x = 7° 29', enables one 

 to gain an idea of the shape of the crystal, while a second 

 orthographic projection which represents the crystal when 

 viewed edgewise, that is so that the base is foreshortened to a 

 line, has been introduced to indicate how very thin the crystal 

 really is. 



For the orthorhombic system, illustrations have already been 

 given of the use of orthographic projections both in drawing 



and in the understanding of the forms of barite crystals. 

 Figure 43 is offered as an additional illustration : It represents 

 a combination observed on brookite from Magnet Cove, 

 Arkansas. From the clinographic projection alone it is very 

 difficult to gain an appreciation of the proportions of the crys- 

 tal ; while the orthographic projection is excellent for showing 

 the distribution of the terminal faces and zonal relations. 

 Figures 44 and 45 represent pyramids of sulphur and octa- 

 hedrite, respectively. Considering the clinographic projections 

 alone, it takes careful inspection to discover any difference 

 between the two figures, while the accompanying orthographic 

 projections indicate at a glance that the pyramid is ortho- 

 rhombic in the one case and tetragonal in the other. 



In the monoclinic system, the clinographic projections alone, 

 figures 46, 47 and 48, need to be supplemented by the accom- 

 panying orthographic projections in order that the real shapes 



