137 
TAB. CCLXIX. 
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 to- 
wards crystallization, if it may be so called, somewhat ac- 
cords 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 for- 
mation 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, 
* A peculiar stone found in columnar forms, reaching many miles, at the 
Giant’s Causeway in Ireland and at many other places, of which more will 
be said hereafter; 
