36 



MINERALOGY 



-a 



Isometric. Draw xy, Fig. 51, the trace of the horizontal plane 

 with the plane of projection, and a'a' at right angles, making 

 o o a', ao == the true 



length of the axis oa. 

 Draw oa' at 18 26' to 

 xy, making oa' = oa ; 

 draw a'a" at right angles 

 to xy; as a' is in front 

 of xy lay off a distance 

 a"a'" = i a'a", draw oa'" 



and extend it to a'"o, 

 c . 



FIG. 52. Axial Cross of Zircon; c = 0.64+ . making a'"o = a' OJ 



then - a"V" will be the 



projection of the axis. The axis at right angles to this is gotten 

 by drawing oai at right angles and equal to oa' ; draw a'iai per- 

 pendicular to xy and make a/'a/ 

 = | a/ai; draw a/'o, extend to 

 a/', making a/'o = oa/', 

 when a/'a/' will be the projec- 

 tion and relative length of the 

 second lateral axis. The three 

 lengths a'"o, a/'o, a'o all rep- 

 resent the same unit of length as 

 measured in turn on each of the 

 axes ; by connecting the extremi- 

 ties of the three axes the clinographical projection of the unit 

 form or octahedron will be obtained, Fig. 49. 



Tetragonal. The tet- 

 ragonal system will not 

 differ from the construc- 

 tion in the isometric, ex- 

 cept that the c axis is 

 not equal to the lateral 

 axes. Draw xy, Fig. 52, 

 and find the projections 

 of the two lateral axes 

 exactly as described in the 

 isometric cross ; at o draw 

 oc = co = oa X (the 

 axial ratio of the mineral 

 FIG. 53. Axial Cross of Apatite; c=o.734+. to be represented), in 



FIG. 52 a. The Unit Pyramid of 

 Zircon ; c = 0.640. 



