106 J. U. n-VEFOKD — CHARACTERISTICS OF MINERALS. 



is of any other fundamental form than the cube, the ray of light 

 traversing it is doubled, or divided into two, and these are polar- 

 ised. That is, they possess properties that are alike in contrary or 

 rectangular positions. This peculiarity is due to the polarisation 

 of light, and cannot be here further noticed.** 



Cleavage. — Somewhat analogous to the subject of crystallisation 

 is that of cleavage. Many minerals split or divide into flakes in 

 various ways, and with greater or less facility. Amongst the 

 easiest to cleave may be mentioned mica, and all degrees of 

 difficulty of cleavage may be observed in other minerals. Some 

 minerals have but one plane of cleavage ; others more, up to six, 

 of which may be instanced the sulphuret of zinc or blende. The 

 cleavage may be parallel to the natural face of the crystal, or 

 otherwise, but always in accordance with a fixed rule. 



Many other peculiarities of structure or texture in minerals may 

 be noticed, all of which requii'e illustration, such as radiated, as in 

 the globular iron pyrites ; massive or amorphous, as in native 

 copper; fibrous, as in asbestos, tremolite, malachite, and satin spar; 

 capillary, as in some specimens of native silver; lamellar, as in mica 

 and talc; stalactitic, as in ordinary stalactites; granular, as in sand- 

 stones ; botryoidal, as in ores of iron and in chalcedony ; and many 

 other features might be adduced to exemplify the varied face of 

 natural productions. 



* This subject has been treated of b)' Mr. Harford in a previous lecture. See 

 ' Transactions,' Vol. I, p. 152. — Ed. 



