Sect. VIIL] 



MINERALOGY. 



243 



r ^ 





tA 



m 



} 



I 



' ft 



\^ 



i 



e\\ 



m 



m 



§ 



{. 



i 



rtltl 



r 



i 





1 



.# 



1^ 



f 



% 



c 



I- 







m 





5 



crystallized minerals he may collect from their crystalline 



forms. 



To cleave minerals, so as to acquire a knowledge of 

 any form made thereby apparent, thus exposing a 

 structure aiding in the determination of the mineral, 

 requires both dexterity and a fair knowled^ 

 bable planes of cleavage. While some minerals, like 

 selenite, cleave most readily, in others it is only by a 

 smart blow upon a chisel, placed in the supposed lines of 

 cleavage, that this is eifected. In others, again, cleavage 



is more to be traced by lines observed on the faces of the 

 crystals than to be obtained mechanically. In some again 

 the structure can only be traced by means of optical 

 researches. It is found that in the same mineral the 

 cleavages are always disposed in the same manner, forming 

 constant angles with each other, and w ith the faces of the 

 crystal When there are three directions of the clea 

 planes, the resulting solid always presents the same angles 



^•r 



agre 





for the same kind of mineral. W 



the planes o 



f 



cleavage are in more than three directions, one set is 

 termed principal, the other supplementary cleavages. 



The crystalline types, as they have been termed, those 

 under which the difterent forms can be classified, h 



H 



ave 



been variously treated by crystallographers. The follow- 

 ing is the arrangement adopted by Dufrenoy, founded on 

 that of Haiiy : 





regii 



With perpendicular axes : 



I. Cube. 



rbomboidal dodecahedron, the hexatetrahedron, the trapezoi- 

 hedron, the octotriahedron, and some other forms. 



II. Right prism with a square base.-The modifications of Trhich are 



the octahedron with a square base, the dioctahedron, and others. 



m2 



