204 PHYSIOGRAPHY. CLASS H. 



Also the results of chemical analysis correspond with 

 this mode of viewing the subject. M. D'AUBUISSON has 

 found one of its varieties to consist of 



Alumina 23-50. 



Silica 48-60. 



Oxide of Iron 11-30. 



Oxide of Manganese 0-50. 



Magnesia 1-60. 



Potash 4-70. 



Water 7'60. 



Carbon 0-30. 



Sulphur 0-10. 



Clay-slate melts into a scoria. It forms rocks, and is asso- 

 ciated and alternates with various other rocks possessing 

 a slaty structure. It occurs in primitive and transition 

 mountains. Sometimes it includes crystals of Chiastolite, 

 more frequently it contains crystals of hexahedral Iron- 

 pyrites. It is spread over many countries as a rock, giving 

 in many places occasion to important mining proceedings. 

 It is used also as a roofing slate, for manufacturing slates 

 for drawing and writing, and some varieties also as whet- 

 stones. It is employed as a flux in melting ores of iron ; 

 but most of its varieties are useless for purposes of build- 

 ing or paving. 



i 



GENUS VI. PEARL-MICA, 

 1. RHOMBOHEDRAL PEARL-MICA. 



Rhomboidal Pearl Mica. JAM. Man. p. 129. Margarita 

 (of Fuchs). PHILL. p. 208. Perlglimmer. LEONH. S. 

 655. 



Fundamental form. Rhombohedron of unknown 

 dimensions. Vol. I. Fig. 7. Simple forms R <x ; 

 P ; P -f oo. Combinations of these with each 

 other. 



