No. 6.J IIARKINGTON — NOTES ON DAWSONlTt!. 307 



2.632. It has a feebly shining lustre, and is slightly translucent 

 on the edges, with a compact or finely granular texture, and an 

 uneven sub-conchoidal fracture. Before the blow-pipe it fuses, 

 with intumescence, into a white enamel. The rock in powder is 

 attacked even by acetic acid, which removes 0.8 per cent, of 

 carbonate of lime, besides 1.5 per cent, of alumina and oxyd of 

 iron ; the latter apparently derived from a carbonate. Nitric 

 acid dissolves a little more lime, oxydises the pyrites, and takes 

 up, besides alumina and alkalies, a considerable portion of man- 

 ganese. This apparently exists in the form of sulphuret, since, 

 while it is soluble in dilute nitric acid, the white portions of the 

 rock afford no trace of manganese before the blow-pipe ; although 

 minute dark-colored grains, associated with the pyrites, were 

 found to give an intense manganese reaction. From the residue 

 after the action of the nitric acid, a solution of carbonate of soda 

 removed a portion of silica ; and the remainder, dried at 300^F. 

 was free from iron and from manganese." 



Na. I. is Dr. Hunt's analysis of the portion insoluble in nitric 

 acid ; No. II. that of the matters dissolved by nitric acid from 

 100 parts of the rock : — 



I. 



Silica 63.25 



Alumina 22.12 



Peroxyd of iron 



Red oxyd of manganese 



Lime 0.56 



Potash 5.92 



Soda 6.29 



Volatile 0.93 



99.07 



The bladedj aluminous mineral alluded to by Dr. Hunt is the 

 Dawsonite of this paper, and will now be described. 



Physical Characters. — Hardness 3. Specific gravity 2.40. 

 Lustre vitreous. Colour white. Transparent — translucent. 



As mentioned above, it is bladed, but the blades show a some- 

 what fibrous structure, which is best seen when fragments are 

 examined under the microscope. With polarized light it exhibits 

 beautiful bands of brilliant colours. As regards its crystalline 

 form I am uncertain, though it is probably raonoclinic, with the 

 inclination of the principal axis about TS*^, 



