Basalt

[continued from tab. 468] shows the probability of this more distinctly, as the substance is found to contain shells such as are allowed to have been inhabitants of a former ocean, and very like the most common Bath Ammonites.

The large specimen with a cast of the Ammonite is by favour of the Rev. Mr. Latrobe, and was accompanied by a very pretty specimen with Pyrites for my own collection. I was favoured with the lower one by Sir Humphrey Davy, who first showed me authentic specimens a few years since. It is the finest grained Basalt I have seen, arid is remarkably neat on two sides, as if it had been part of a column, and breaks partly into laminæ perpendicular to the sides. The shelly impression is probably the same as the other, only smaller. The Iron, where the shell has left the impression, seems re-oxidized by exposure, if I may use the expression, I do not at present know of any other shells in Basalt, and these only impressed.

Some Basalt is attractible to the Magnet; but this character varies from the quantify and state of the Iron, as neither of these specimens are.

It is coarse or fine grained, with minute or conspicuous illinitions; compact or porous, or holding different substances in vesicles analogous to Toadstone. The fracture of the large-grained is conchoidal; of the fine-grained it is splintery. It is very hard and toughish within, but externally it may be scratched with a knife, so that people mark their names on the pillars at Antrim, Colour black or dark grey changing to ferruginous by exposure, apparently slowly. It melts easily, forming a black semitransparent glass if quickly cooled; but if slowly cooled it assumes nearly its former appearance, in the same manner as bottle glass does that of calcined flint from which it was made.

Spec Grav, from 2.864 to 3.000 according to different authors.