38 



MINERALOGY 



jection of a it is necessary to understand the position of the angle 

 the position of ft is always back, Fig. 57, of the plane of pro- 

 jection, here represented by cc; 

 its value is always taken less than 

 90. Therefore the angle be- 

 tween the c and a axes in front 

 will be greater than 90, in this 

 case 106 2', and the extremity 

 of the a axis in front will be be- 

 low the horizontal plane a dis- 

 tance oa'", depending upon the 

 value of the angle ft and the 

 length of the a axis. This dis- 

 tance is found by drawing aoc 

 = ft and extending in the direc- 

 tion of a, making oa = ob X 9-55 + ; 

 draw aa"' at 9 28' with the hori- 

 zontal, then the projection of the 

 end of the clinoaxis in front will 

 due to the angle ft + A" A'" 

 Therefore lay out in Fig. 58 



FIG. 56. The Unit Pyramid of Barite. 



appear below xy a distance oa"' 



(i aa") due to the angle 9 28'. 



the angle aoa' = 16 2' and 



oa' = the unit on the a axis, 



then the extremity of the a 



axis will appear below xy a dis- 

 tance aa' due to the angle ft, 



then by the revolution of 



18 26' and the elevation of 



the eye 9 28', the projection of the extremity will fall under a'", 



a distance aa'" = aa' + 1 a"a'"; connect a with o and extend to & 



an equal distance, 

 when aa will be the 

 projection of the clino- 

 axis; connecting the 

 extremities of the 

 projected axes, when 

 Fig. 59 will represent 

 the unit pyramid of 

 amphibole. 



Triclinic. Let it be 

 FIG. 58. Axial Cross of Amphibole. required to draw the 



