DOMAIN I. SIDEROUS. 



On the passage of Simplon an inter- 

 mediate slate, between the mica and the common, 

 of a brilliant and undulated appearance, contain* 

 ing garnets. 



MODE XIII. MICA SLATE. 



Arrangement. It is difficult properly to arrange mica slate. 

 Though it contain a great quantity of quartz, it 

 has always been classed among the argillaceous 

 substances, as the mica is the chief character- 

 istic. Mica sometimes contains no magnesia; 

 but according to the analyses given by Haiiy, 

 the brown, grey, or black, which are the most 

 abundant and common in mica slate, contain a 

 greater portion of iron than of argil, the quan- 

 tity of potash being also considerable. Bergman 

 found 9 parts of iron in mica; Kirwan nearly 

 20 : even of the colourless kind Klaproth dis- 

 covered 15 in one sort, and 22 in another. 



Connexions. Mica slate has also a natural connexion with 

 common slate, into which it often passes*. It 

 must also be observed that Saussure found in the 



* Daubuisson's curious and elaborate analysis (Jour, de Ph. 

 180Q) proves, that the composition of mica slate and slate is identi- 

 cally the same. The mode forms the only difference. 



