40 



it occurs in the Trap rocks which compose the principal part of 

 that Island, or derives its origin from the dykes in the red Sand- 

 stone or from rounded pebbles in the conglomerate. 



It occurs in roimded masses of a jet black color, and is por- 

 phyritic, with crystals of glassy Feldspar, and occasional amyg- 

 dules of carbonate of lime, surrounded by a red jaspery crust. 

 It is black and shining like obsidian, but has a more brilliant 

 lustre. 



Fracture conchoidal. Hardness, 5.5. Specific gravity, 

 2.375. Before the blowpipe it swells very much, exfoliates, and 

 becomes ash-gray. At a high temperature on charcoal it melts 

 into a pale green blebby glass. In a tube the mineral gives out 

 by heat, pure water only. 



Chemical analysis on two grammes of the mineral, dry at 

 212° F. gave, per cent. 



Silica, 67.90 



Alumina, 11.20 



Per Oxide of Iron, 6.40 



Lime, . . . . . . . .3.10 



Oxide of Manganese, .... 0.80 



Soda, 2.61 



Water, 8.00 



100.01 



Comparing this analysis with those given in Dufrenoy's Min- 

 eralogy, tom. iii. p. 358, and in Dana's Mineralogy, p. 329, it 

 will be seen that our mineral is very similar in composition to 

 the Pitchstone of the Island of Arran. 



Dr. C. T. Jackson also communicated the results of his 

 analysis of a crystal of Phosphate of Lime, from Hurds- 

 town, New Jersey, discovered by Mr. Alger and himself. 



The crystal was of a fine yellow color and possessed a 

 resplendent lustre on the surface. 



Its specific gravity is 3.205. Hardness, 5. Does not phos- 

 phoresce when thrown on heated iron. 



By qualitative analysis it was found to consist of Phosphate of 

 Lime, Chloride of Calcium, and Fluoride of Calcium, the color 

 being due to a little per oxide of Iron and Manganese. 



