COMPOSITION OF MICA AND UORNBLENDE. 259 



in large masses, and is of various colours — white, grey, brown, green, 

 and black. It is soft and readily cut with a knife. The thin shining 

 particles that occur in many sand stones, and especially between the 

 partings of the beds, and give them what is called a micaceous charac- 

 ter, are only more or less weathered portions of this mineral. 



Mica also consists of silicates, though its constitution is not always so 

 simple as that of felspar. In some varieties magnesia is present, whilst 

 in others it is almost wholly wanting, as is shewn by the following com- 

 position of two specimens from different localities. 



Potash. Magnesian. 



Mica. Mica. 



Silica 46-10 40-00 



Alumina .... 31-60 12-67 



P rot-Oxide of Iron . 8-65 19.03 



Magnesia .... — 15-70 



Potash 8-39 5-61 



Oxide of Magnesia . 1-40 0-63 



Fluoric Acid . . . 1-12 2-10 



Water ..... 1-00 Titanic Acid 1-63 



98-26 97-37 



If we neglect the three last substances, which are present only in small 

 quantities, and recollect that the silica is in combination with all the 

 other substances which stand beneath it, we see that these varieties of 

 mica consist of a silicate of alumina, combined in the one with silicate 

 of iron and silicate of potash, and in the other with silicate of iron and 

 silicate of magnesia. 



4°. Hornhleiide occurs of various colours, but that which forms a con- 

 stituent of the syenites and of the basalts is of a dark green or brownish 

 black colour, is often in regular crystals, and is readily distinguished 

 from quartz and felspar by its colour, and from black mica by not split- 

 ting into thin layers, when heated in the flame of a candle. It consists 

 of silicates of alumina, lime, magnesia, and oxide of iron, or per cent, 

 f.f— 



Basaltic Syenitic 



Hornblende. Hornblende. 



Silica ..... 42-24 45-69 



Alumina .... 13-92 12-18 



Lime 12-24 13-83 



Magnesia .... 13-74 18-79 



Prot-Oxide of Iron . 14-59 7-32 



Oxide of Manganese 0-33 0-22 



Fluoric Acid ... — 1-50 



97-06 99-53 



A comparison of these two analyses shows that the proportions of 

 magnesia and oxide of iron sometimes vary considerably, yet that the 

 hornblendes still maintain the same general composition. They are re- 

 markably distinguished from felspar hy the total absence of potash and 

 soda, and by containing a large proportion of lime and magnesia. From 

 the potash-mica they are distinguished by the same chemical differen- 

 ces, and from the magnesian mica by containing lime to^the amount of 



