S°8 
Mr. Watt’s Observations 
and the prisms might thus shoot to an indefinite length into the 
undisturbed central mass of the fluid, till their structure was 
deranged by the superior influence of a counteracting cause, 
which would be provided by the action of the atmosphere on the 
upper surface of the basalt. If this arrangement existed, the 
same cause that determined the concentric fractures of the fibres 
of the spheroids, would produce convex articulations in the 
lower joints of the prisms ; and, in proportion as the centre 
from which they were generated became more remote, the arti- 
culations w'ould approximate to planes. If the generating centres 
were not equidistant, the forms of the pillars would be irre- 
gular; and the irregularity would be in proportion to the 
diversity of distance between the centres. If the difference was 
great, the number of sides would be altered, and they might be 
found pentagonal, tetrahedral, and trihedral. As the compres- 
sion of the fibres would be greatest in the level of the generating 
centres, the lower part of the prisms would be most compact. 
All these conditions seem to be fulfilled, in the actual con- 
formation of basaltic columns ; for, in every instance I am 
acquainted with, they appear to have been formed in the tranquil 
bosom of the mass, as they have been originally masked by 
amorphous trap, and their prismatic structure is only displayed by 
the removal of this covering. This has been variously effected, 
sometimes by the apparent disrupture of rocks, sometimes by the 
exterior portions of the mass being thrown down by the failure 
of the ground on which it stood, sometimes by the violence of 
the waves, and not unfrequently by the working of quarries. 
In most instances, these operations have only removed the 
covering from one side of the colonnade; and it remains crowned, 
and generally surrounded, by an immense amorphous mass. 
