56 TERMINOLOGY. 



OF THE IMPERFECTIONS OF CRYSTALS IN RESPECT TO 

 THEIR FORM. 



. 52. KINDS OF IMPERFECTION IN FORM. 



The irregularities noticeable in crystals are of two kinds; 

 and originate either in the formation of the crystals them- 

 selves, or they are the consequence of the contact of these 

 with other minerals. 



In the foregoing considerations of forms, the student may have 

 been led to imagine that crystals uniformly occur under planes of a 

 constant f.gure and extent. He now needs to be informed that per- 

 fect regularity, in these respects, is rarely to be found. At the same 

 time, the deviations from it, (as will presently be seen,) are of such 

 a kind as to occasion only a slight inconvenience in ascertaining their 

 relations, and too unimportant to require that they should be treated 

 of, except under the idea of perfect regularity, 



In those cases where crystals are possessed of the irregularity al- 

 luded to, and where the circumstances under which they are found 

 do not indicate any external disturbance of their forms, we are natu- 

 rally led to suppose that the deviations it presents are founded upon 

 the formation of the crystals themselves. On the other hand, where 

 crystals occur in contact with each other and with other minerals, 

 or where they have been subjected to other accidents, their irregu- 

 larities are referrible to these causes. The former imperfections are 

 the most important. The latter will be more particularly treated of 

 under compound minerals. At present, it is only necessary to ex- 

 amine in what shape an individual will appear, which is prevented 

 from assuming its regular form by some external obstacle. 



. 53. IRREGULARITIES DEPENDING UPON THE FORMATION 

 OF THE INDIVIDUALS THEMSELVES. 



Deviations from regularity in crystalline forms appear 

 either in their size and figure, or in the physical quality of 

 their faces. 



