Prismatic Plumbago

For this fine specimen we are obliged to the favour of G. B. Greenough, Esq., M.P. P.G.S., who had it from near Crumnock in Ayrshire, The shape and approach towards crystallization, if it may be so called, somewhat accords with some of the columnar stones^*, of which the late Mr, Watt has with much philosophy given a valuable paper in the Philosophical Transactions for 1804. The formation of Basalt differs much from this, in which we have to account for the transverse joints or articulations, which do not so plainly appear in this; although there are some indications of separation in some parts, yet I would rather compare it to the separation usual to crystals of coals.—See Crystallography and British Mineralogy, tab, 48. The same form nearly remains even in the cinder of the common coal, if slowly burnt; so this resemblance may depend upon similar laws, having the same original atom or nucleus, if I may so call it.

This extraordinary substance is as yet new in this form to the mineralogical world, nor do we know of it as any other than a British production.

The specimen before us shows the probable varieties of this formation, from straight to curved, and are somewhat varied in the angles and number of them, in which it much resembles various Coals when partly burnt in a slow fire, which crack or divide into irregular columnar appearances, although larger and more expanded, somewhat agreeing also with the mineral coke—see tab. 192:—hut as they are also more compact, it may depend upon the substances around them preventing in a great measure their expansion, which would agree in some measure with the experiments made by the ingenious Sir James Hall.

A long time since I received a portion of columnar Glance Coal from a Dyke about ten miles from Wanlock-Head, by favour of G. Laing, Esq., which so nearly resembles this in structure and appearance, that thence we might suppose them the same thing; and Mr. Greenough has specimens which are more intermediate, some parts having Plumbago about them.