Measurement and Cleavage of certain Primitive Crystals. 247 



blow with the hammer, even if the edge of the knife be precisely 

 along the natural joints, is apt to have the effect of splitting it in 

 various directions. 



Quarks, 



Fig. 5. 



Since the time of presenting to the notice of the Geological 

 Society, the memoir on the measurements of the angles of the 

 primitive crystals of quartz and the sulphate of barytes, several other 

 crystals of quartz with perfectly reflecting planes have corroborated 

 the opinion therein stated, that the angles of the primitive rhomboide, 

 which is slightly obtuse, are 94° 15' & 85° 45'; which have also 

 been further strengthened, and I may say confirmed to be their 

 true value by co-incidences obtained by means of the reflecting 

 goniometer from some fragments, exhibiting brilliant planes parallel 

 with those of the rhomboide. 



Crystals of quartz do not often present clear indications of their 

 natural joints. By consulting Haiiy, Traite PI. XL. fig. 3. it will 

 be observed that the primitive rhomboide is so situated in a dode- 

 cahedral crystal, that six of the twelve planes of the latter figure 

 are alternately parallel with the primitive planes ; the other six 

 being the result of a modification explained by fig. 2 of the same 

 plate. If therefore we would cleave a prismatic crystal of quartz, 

 we are by the above circumstance assured, that by striking the 

 prism diagonally and parallel with any plane of the upper or lower 

 pyramid, it will be parallel with one or other of the planes of 

 the primitive rhomboide, and, of course, in the direction of its 

 natural joints. It will be well to attend to this observation, if 



2 i 2 



