GUIDE TO THE MIXERALOGIC COLLECTIONS 



21 



For each tetragonal species there may be several pyramids 

 of the first order intersecting the vertical axis at multiples or 



Fig. 68 Fig. 69 



fractions of the unit length c and producing steeper or flatter 

 forms than the unit pyramid. 



The pyramid of the second order is composed of eight isosceles 

 triangles each of which is parallel to one horizontal axis a and 

 intersects tho second horizontal axis a and the vertical axis c. 



The second order pyramid of zircon is shown in fig. 70. 



Fig. 70 Fig. 71 



The ditetragonal pyramid (fig. 71) is composed of 16 isosceles 

 triangular faces intersecting the horizontal axes at unequal dis- 

 tances and also intersecting the vertical axis. 



Prisms. For each type of pyramid in th^ normal group there 

 is a corresponding prism having the same relative intercepts 

 on the horizontal axes as the pyramids of the same name, and 

 having every face parallel to the vertical axis. These prisms 

 are denoted as follows: 



Prism of the first order, having four faces, represented in 

 fig. 72 by the faces marked m. 



