ALUMINA. 367 



PITCHSTONE. 



[From its resemblance to some varieties of pitch.] 



Pitchstone. Cleavdand, Phillips and Skepard. — Petrosilei, Resinite. BaUy. — Variety of Q,uaiz. Beudant. — 

 Fusible ftuartz, Obsidian, Pitchstone, Peailstone, Pvimice. Jameson. — Empyrodoier (^uarz. Mohs, 



Description. Colour grey, blue, green, yellow, brown and black, but usually dull. Streak 

 white. It is destitute of regular form or cleavage ; occurs massive, vesicular and porous. 

 Fracture conchoidal, sometimes imperfect. Lustre vitreous, resinous and pearly. Varies 

 from transparent to opaque. Hardness from 6.0 to 7.0. Specific gravity from 2.21 to 2.39. 

 Before the blowpipe, the different varieties of this mineral melt with various degrees of facility, 

 into a vesicular glass or enamel. 



Varieties. Obsidian. This has a splendent vitreous lustre and conchoidal fracture. 



Pitchstone. A variety with a shining and resinous lustre, and translucent on the edges. 



Pearlstone. This name is applied to the varieties which occur in globular, pearly, trans- 

 lucent concretions. 



Pumice. White and grey, vesicular and fibrous, with a pearly lustre, and often so light as 

 to swim on water. 



Composition. Obsidian — Silica 78.00, alumina 10.00, lime 1.00, potash 6.00, oxide 

 of iron 3 . 60 ( Vauquelin). 



Pitchstone — Silica 72 . 80, alumina 1 1 . 50, soda 2 . 85, lime 1 . 20, oxide of iron 3 . 03, water 

 and bituminous matter . 50. 



Pearlstone — Silica 77.00, alumina 13.00, potash 1.50, lime and soda 2.70, oxide of iron 

 and manganese 2 . 00, water 4 . 00 ( Vauquelin). 



Pumice — Silica 77.50, alumina 17.50, soda and potash 3.00, oxides of iron and manga- 

 nese 1.75 {Klaproth). 



Geological Situation. Pitchstone, in all its varieties, is usually found in volcanic dis- 

 tricts, or forming beds or veins in rocks of igneous origin. 



LOCALITIES. 



Essex County. This mineral is found in the trappean dykes of Essex and Warren coun- 

 ties. The colour is black or bluish black. It has a vitreous lustre, and fuses alone into a 

 black globule. 



Some years since. Dr. G. Troost described a mineral which belongs to this species, found 

 by Lieut. Bayfield on the north shore of Lake Superior, near the island of St. Ignatius. It 

 was adhering to one of the quadrilateral columns of the trappean dykes which so frequently 

 traverse the hornblendic granites of that region. This mineral is opaque, has a perfectly 

 black colour and small conchoidal fracture, a resinous lustre, and a hardness sufficient to 



