Mr. Watt’s Observations 
SV> 
intermedium occurred, there was invariably a division in the 
middle of the vein. Not unfrequently, the veins contain three 
sets of prisms ; a range of small ones on each side, and of 
much larger ones in the middle. In this case, the little prisms 
are always separated from the large ones, and the divisions of 
the large ones are very irregular.* 
After the statement of my opinion, that perfect similarity of 
structure may exist in the products of aqueous and igneous 
formation, it will hardly be necessary to conclude these obser- 
vations with remarking, that I should not consider the estab- 
lishment of these peculiar modes of arrangement as the slightest 
demonstration of the igneous origin of basalt. It appears to 
me, that the truth of my deductions is entirely independent of 
either theory, and that, if ever the period should arrive when the 
origin of basalt shall be determined by irrefragable demonstra- 
tion, the inferences I have drawn may be accommodated with 
equal facility to either mode of agency.-f 
* The observations alluded to were made during the course of last Summer, (1803,! 
on the very numerous basalt veins, or, as they are there called. Whin Dykes, which 
traverse the red sandstone and red sandstone breccia, which forms the greatest part of 
the coast of the Firth of Clyde, between Greenock and the Largs. 
+ Mr. Keir, in his Paper on the Crystallizations formed in Glass, suggests the pro- 
bability of basaltic pillars being formed by the crystallization of vitreous lavas. See 
Philosophical Transactions for 1776, Vol. LX VI. page 530. 
Dolomieu was of opinion, that the prismatic form was peculiar to lavas which had 
flowed into the sea ; and he attributed it to the shrinking of the mass : his description 
of the appearances exhibited by what he calls the prismatic lavas at the foot of Etna, 
merits quotation. 
“ In the lavas of Etna, the form and dimensions of the columns vary as much as 
" the manner in which they are grouped ; hexaedral and pentaedral prisms are most 
“ abundant ; then the tetraedral, the triedral, heptaedral, and octaedral. The least 1 
“ have seen are only four inches diameter ; others are more than three feet ; they are 
