GUIDE TO THE MINERALOGIC COLLECTIONS 



33 



Domes. Domes may be considered horizontal prisms; they 

 are parallel to one horizontal axis and cut the other 

 horizontal axis and the vertical axis. Like the pinacoids they 

 take their names from the axes to which they are parallel thus : 



The macrodome (fig. 125) is parallel to the macroaxis and 

 cuts the brachy and the vertical axes. 



The brachydome (fig. 126) is parallel to the 

 brachyaxis and cuts the macro and the vertical 

 axes. 



Macrodomes and brachydomes are often re- 

 peated in series, the relative value of the vertical 

 intercepts being subject to the law of rational 

 indexes. 



Pyramids. The single type of pyramid found 

 in this system is shown in fig. 127 and is com- 

 posed of eight faces each cutting all three axes. Fig - 127 

 The unit pyramid for any species intersects the axes at 

 distances corresponding to the axial ratio for the species. 



Fig. 131 Fig. 132 Fig. 133 Fig. 134 



As in the case of the prisms there are macro and 

 brachy pyramids bearing relations to the unit pyramid analo- 



