218 TERMINOLOGY. . 160. 



termed implanted crystals. Implanted crystals are always 

 incomplete ; because those parts are wanting in which the 

 crystals are attached to the supporting mass. They can- 

 not be removed from it, so as to leave behind a print of 

 their form; they only may be broken off from the support 

 with which they cohere more or less firmly. 



Implanted crystals must be duly completed, for the 

 purpose of a crystallographic consideration ; so must also 

 those crystals, which, by some accident, or on purpose, have 

 been broken or rendered incomplete. The only rules we 

 must attend to in this process are those of symmetry, by 

 which a perfect equality and similarity is established as to 

 the number and situation of f?_es between those parts of 

 the crystal which are wanting, and those which may be ob- 

 served. The most common crystallisation of rhombohedral 

 Quartz, consists of an isosceles six-sided pyramid, which is 

 combined in a parallel position with a regular six-sided 

 prism. As these crystals very often occur implanted, the 

 observation of one end of the pyramid only is possible ; 

 evidently the opposite termination of the crystal must be 

 completed, by supposing it equal and similar to that which 

 has been observed. Simple pyramids of rhombohedral 

 Quartz, (and in similar cases also the forms of other mine- 

 rals), if they present only one of their apices to the ob- 

 Lsrver, must likewise be completed according to the rules 

 of symmetry ; and we can never be entitled to assume or 

 consider such things as simple pyramids, because those do 

 not exist among the productions of nature, nor are they 

 obtained from the different processes of derivation (. 80 

 83.). Similar examples occur in pyramidal Garnet, in octa- 

 hedral Fluor-haloide, in prismatic Hal-baryte, &c. ; which 

 must be completed according to the method explained 

 above. 



There are cases, however, in which it becomes necessary 

 to allow of an exception of that rule. These comprehend 

 the crystals, in which two opposite solid angles possess a dif- 

 ferent configuration (. 147-)- Evidently this difference 

 always must remain within the range of the series of crys- 



