3 12 
Mr. Watt's Observations 
be such, are prismatic.* Columns of porphyry are not rare; 
and, among other places, are found near Dresden, several feet in 
length, and not more than two inches in diameter. Columns 
of petrosilex compose a large portion of a mountain near 
Conistone lake. Very perfect quadrangular prisms of argillaceous 
schistus are found near Llanwrst. Rubble slate assumes the co- 
lumnar form at Barmouth. The limestone near Cyfartha, in 
Glamorganshire, is divided into very regular acute rhomboidal 
prisms : even the sandstone of the same district is not unfre- 
quently columnar; and one of the beds of gypsum at Mont- 
martre is distinctly divided into pretty regular columns. Sand- 
stone, clay, argillaceous iron ore, and many other substances, 
become prismatic by torrefaction ; and the prisms of starch 
formed in drying have often been considered as illustrative of 
basaltic formations. 
I am very far from conceiving, that all these configurations 
are influenced by such systematic arrangements as have deter- 
mined the form of some basaltic columns. I consider most of 
them as solely attributable to contraction; which is only a farther 
* Almost all the prisms at the foot of Etna, described by Dolomieu, are of 
dubious origin ; most of them are probably basalt. The columns of the Vicentine 
are of the same substance, and so are the prismatic lavas of Auvergne, and of the 
Vivarais. The bed of lava at la Scala, near Portici, is divided by vertical fissures, 
which give it the aspect of irregular columns. At Aquapendente, in a quarry of un« 
doubted lava, near the road, are some much more perfect prisms; but the most 
beautiful I have seen, are the small ones from Ponza. The columns at Bolsena, are 
said to be basalt. Those of the Euganean hills are very irregular in form ; in their 
texture they are certainly wholly unlike granite, which Mr. Strange thought they 
resembled. I believe them to be lava. 
The mention of some columnar formations that follows, is by no means intended as 
an enumeration of them. I have confined myself to those which I have either 
inspected in their natural situation, or of which I have seen numerous specimens. 
