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Such media are the volcanic rocks when they are rapidly cooled, producing various kinds of pitchstone, obsidian, &c.

There are numerous varieties of lava, such as basalt, obsidian, pitchstone, pearlstone, trachyte, &c.; some are heavy compact rocks, others are light and porous.

The walls, or banks, of the channel have been denuded away, thus converting the pitchstone casting into a projecting wall of rock.

The lavas comprise dark pitchstone, resembling that at Kirk Yetholm, and porphyritic and amygdaloidal andesites and basalts.

If pumice was to be found anywhere in Scotland, we might a priori expect to find it in connection with by far the largest mass of pitchstone in the kingdom.