Dr. Thomson on some New MhieraHs. 68 



sent me also from Perth, in Upper Canada, by Mr. Wilson, and also 

 by Dr. Holmes of Montreal, under the name of iridescent felspar ; but 

 neither its characters, nor its composition correspond with that 

 appellation. 



The specimens were amorphous masses, and had the appearance of 

 having constituted part of a rock blasted by gunpowder. 



It is light brownish-rod, and exhibits a play of colours chiefly blue 

 on the surface. It is translucent on the edges. The lustre is vitreous, 

 and the texture imperfectly foliated. Its hardness is only 3*75, which 

 is a good deal loss than that of felspar. 



Its specific gravity is 2-5 08. 



Before the blow-pipe it becomes white, but does not melt. With 

 carbonate of soda it melts into a green coloured bead ; and, on adding 

 nitre, the colour becomes red; with borax, it fuses into a colourless 

 bead. 



Its constituents were found to be — • 



Silica, 



Alumina, 



Potash, 



Lime, 



Magnesia, 



72-35 

 7-60 



1506 

 1.35 

 1-00 



Oxides of iron and manganese, 1*25 

 Moisture, .... 0*50 



99-11 



The silica is much greater than in felspar, and the alumina much 

 less, while the proportion of potash is nearly the same. If we were to 

 consider the lime and magnesia, and the oxides of iron and magnesia, 

 as accidental bodies united to silica, in the same ratio as the alumina 

 and the potash, the constitution of the mineral might be represented 

 by 4 (AIS*) 4- 3 (KS^). If the lime and magnesia be essential con- 

 stituents, the formula will be AIS'' + (g K + j^ cal. + J Mg.) S*. 



4. Silicite. — The fourth mineral which I shall notice, I have distin- 

 guished by the name of silicite, from the great resemblance which it 

 has to quartz, in its external aspect, though it differs entirely from 

 that mineral in its constitution. It occurs in a basaltic rock in the 

 county of Antrim, and was given me by Mr. Doran, an Irish mineral 

 dealer. 



The colour is white, with a shade of yellow, the texture foliated, and 

 the fracture small conchoidaL Its lustre is vitreous, its hardness 

 nearly the same as that of quartz, and its specific gravity 2.666, or 

 nearly the same as that of rock crystaL 



With carbonate of soda, it fuses into an opaque bead, and with borax, 

 into a transparent colourless bead. Its constituents arc, 



