230 -* TEEMINOLOGY. . 169. 



ately from . 166. For, if the faces containing a form of 

 cleavage are parallel to the faces of a crystalline form in 

 the same species, the form obtained by cleavage must itself 

 necessarily be similar to the form of crystallisation ; it must 

 possess the same dimensions and relations, and therefore 

 be capable of being substituted for the member in the series. 

 This is applicable to both simple and compound forms of 

 cleavage ; because nature produces combinations of such 

 simple forms only as are members of the same series (. 139.). 

 Cleavage therefore extends the application of crystallo- 

 graphy to the productions of the mineral kingdom, and 

 enables us not only to determine the system, but also 

 very often even the series of crystallisation of such indivi- 

 duals or species, in which crystals are either not known at all, 

 or at least are not the immediate object of our observation. 

 Hence the study of cleavage is particularly recommended 

 to those who intend to apply the Characteristic to nature, 

 and to acquire that degree of skill which is required for 

 determining .with facility and certainty the productions of 

 the mineral kingdom by the assistance of the Characteristic. 



An accurate knowledge of the peculiarities of cleavage is 

 moreover very useful for recognizing and completing such 

 crystalline forms as occur indistinct, imperfect, cohering 

 with others, &c. Some single faces, fissures in the interior, 

 strise arising from the superposition of laminae and other 

 observations of that kind, are very often sufficient for as- 

 signing the true position to a crystalline form, and by this 

 means to acquire a correct knowledge of its nature. 



The preceding observations indisputably shew, that cleav- 

 age in itself is a highly remarkable phenomenon of in- 

 organic nature. Yet this will appear still more strikingly, 

 if we consider it in connexion with the forms of crystallisa- 

 tion, in as far as they both refer to the natural-historical 

 species. If we attend also to the less apparent directions 

 of cleavage, or those faces which are less distinct, though 

 subject to the same laws, we are led to the conclusion 

 that cleavage represents the phenomenon of crystallisa- 

 tion to its full extent. The Jattcr is therefore not a 



