SYSTEM OF CRYSTALLIZATION. 59 



We say crystals which are formed in an empty space, this is not ne- 

 cessary provided the mineral 4nto which the crystal shoots from its 

 support is different from the crystals themselves, and from the sup- 

 port generally, and is capable of being detached from the crystals 

 so as to leave them free except in their attachment to the supporting 

 mass with which their connexion is peculiar, inasmuch as they can- 

 not be removed from it so as to leave behind a print of their form ; 

 they can only be separated from the support by breaking. Of course, 

 implanted crystals are always incomplete, because those parts are 

 wanting in which the crystals are attached to the supporting mass. 



Other imperfections to which crystals are liable from external 

 sources, are such as arise from disturbances in the rocks which con- 

 tain them, in consequence of which they are contorted or have 

 been subject to slips ; or from their having been acted upon by heat, 

 and thus become rounded on some of their edges and angles. 



Implanted crystals as well as those which are irregular from the 

 -undue enlargement of some of their planes, or which have been 

 broken by accident, are completed according to the rules of sym- 

 metry in order to fit them for the purpose of crystallographic con- 

 sideration. For example, Quartz ordinarily occurs in regular six 

 sided Prisms, terminated at each extremity by six-sided pyramids. 

 But when these crystals occur implanted, they are usually attached 

 to the supporting mineral by one end of the prism without the in- 

 tervention of the pyramid ; we therefore complete this termination, 

 by supposing it equal and similar to that which has been observed. 

 Sometimes only one pyramid is observable among implanted crys- 

 tals of Quartz : in such a case, we have to imagine the prism and 

 other pyramids conformably to the rules of symmetry; for we can 

 never be entitled to assume or consider such crystals as simple pyr- 

 amids, because such forms do not exist among minerals, nor are they 

 capable of being obtained by any process of derivation. 



A few cases however exist, in which it is necessary to allow of 

 exceptions to this general rule. Such are the crystals whose oppo- 

 site solid angles possess a different configuration, and which present 

 differences in their electric action when heated, . 50. p. 47. 



55. SYSTEM or CRYSTALLIZATION. 



The assemblage of forms derivable from one primary 

 form, is termed a system of crystallization. 



