284 Intelligence and Miscellaneous Articles. 



days over sulphuric acid in the exhausted receiver ; nor is it de- 

 stroyed when the quartz is digested, either hot or cold, in hydro- 

 chloric acid or ammonia : this resistance to chemical and physical 

 agents is probably derived, in part, from the intimate admixture of 

 the organic matter in the pores of the quartz. 



Orthose. — The colour of this is generally white or grayish-white, 

 sometimes, however, it is fawn- or rose-coloured, or pale scarlet. 

 When it has a tendency to a brownish-yellow or red colour, the 

 mineral is altered by incipient decomposition : it has a brilliant 

 pearly lustre. 



According to Saussure, its density is 2'6i5. Its crystals are often 

 several centimetres long, well-formed, and generally possess the 

 characteristic made which is common to them in granitic rocks. 



The crystals of orthose, oligoclase and mica analysed by M. 

 Delesse, were taken from an enormous block, well-known at Cha- 

 mouni, and which has fallen from the needles above the Mer de 

 Glace. 



The following was found by the author to be the composition of 

 the grayish-white orthose, with a tint of fawn colour: — 



Silica 66-48 



Alumina 19'06 



Lime 0-63 



Peroxide of iron traces 



Magnesia traces 



Potash 10-52 



Soda 2-30 



98-99 



Oligoclase. — In protogine, as in the greater number of granites, 

 there is besides orthose a second felspar, which in this case is oligo- 

 clase. It is somewhat difficult to distinguish on account of its 

 white colour, which is nearly the same as that of orthose : this 

 however is translucent, whilst oligoclase is dull, or very slightly 

 greenish ; it is, moreover, characterized by parallel microscopic 

 striae, and the crystals are often complex and macled, like those of 

 the albite of Carlsbad. Its density is 2-633. 



The analysis of very pure crystals from the needles of the Mer 

 de Glace, made by carbonate of soda and hydrofluoric acid, gave — 



Silica 63-25 



Alumina 23-92 



Peroxide of iron traces 



Oxide of manganese .... traces 



Lime 3*23 



Magnesia 0-32 



Soda 6-88 



Potash 2-31 



99-91 

 This composition is almost identical with that of the oligoclase of 



