Page:Transactions of the Geological Society, 1st series, vol. 3.djvu/425

 oblique octahedron, more or less complete, from the size of a pin's head to that of about three lines across. Very often the crystals are so much compressed that they exhibit to the naked eye only two large faces, the lateral ones being then almost imperceptible. Sometimes the crystals are grouped two or three together, and striped parallel to the great diagonal.

The external lustre is accidental, being mostly hid by the fine powder of the matrix covering the surface, but in a fresh fracture the internal lustre is very brilliant.

The cross fracture is uneven, though approaching to the laminar. The longitudinal fracture is perfect laminar, in which direction it also can be easily divided.

The small fragments are in general undetermined, more or less pointed, often taking the form of a rhomboidal prism, the four faces of which exhibit a specular gloss.

It is semitransparent, though not so much as French flint.

Its hardness is the same as that of corundum from India.

It breaks easily in the direction of the laminæ, but in other directions not without some difficulty.

The specific gravity of two specimens was 4.20 and 4.02.

At the blowpipe it cannot be melted, neither alone nor with borax, &c.

Its most common and immediate matrix is a light grey coloured laminar iron ore, consisting of an intimate mixture of red and black oxide of iron, without any visible earth. This ore (a variety of eisenglanz, Werner,) which has been mentioned by Count de