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BASALT. 
  
Birmingham for converting this rock into an opaque 
glass for various economic uses. 
The remarkable columnar structure which B. fre- 
quently assumes, is its most striking characteristic. 
The columns vary in the number of their angles from 
three to twelve; but they have most commonly 
from five to seven sides. They are frequently 
divided transversely by joints at nearly equal dis- 
tances. The direction of the columns is always at 
right angles to the greatest extension of the mass, 
o that when B, occurs as a bed, either overlying, or 
interstratified with the regular sirata, the columns 
are perpendicular, while they are horizontal when 
the B. exists as a dike. 
The columnar structure was at first believed %o 
be owing to a modification of the crystalline force. 
Such a supposition was favoured by the external 
form of the columns; but the total absence of inter- 
nal structure shewed that the explanation must be 
sought elsewhere. In 1804, Mr Gregory Watt pro- 
pounded a theory of the origin of the structure, 
ascribing it to the pressure of numerous spheres 
  
Fingal’s Cave. 
m each other, during the process of cooling, such 
spheres being produced in planes of refrigeration 
or absorption. They increase by the successive 
formation of external concentric coats, until their 
growth is prevented by the contact of neighbouring 
spheres ; and as in a layer of equal-sized spheres, 
each is pressed on by six others, the result is that 
each sphere will be squeezed into a regular hexagon. 
Watt published this theory as the result of his 
celebrated observations on the cooling of a mass of 
molten basalt, in which he noticed the production 
of numerous spheroids, having a radiate structure. 
Many greenstones, in weathering, present such a 
structure, giving often to the rock the appearance 
as if it were composed of a mass of cannon- 
balls, and Watt’s experiments satisfactorily explain 
this phenomenon; but they will not go further. 
Anxious, however, that they should throw some 
light on the structure of basaltic columns, he 
manages it by the following remarkable assump- 
tion: ‘In a stratum composed of an indefinite 
number in superficial extent, but only one in height, 
of impenetrable spheroids, with nearly equidistant 
centres, if their peripheries should come in contact 
in the same plane, it seems obvious that their 
mutual action would form them into hexagons; and 
if these were resisted below, and there was no 
opposing cause above them, it seems equally clear 
that they would extend their dimensions upwards, 
and thus form hexagonal prisms, whose length might 
be indefinitely greater than their diameters. The 
  
further the extremities of the radii were removed 
from the centre, the greater would be their approach 
to parallelism; and the structure would be finally 
propagated by nearly parallel fibres, still keeping 
within the limits of the hexagonal prism with which 
their incipient formation commenced; 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” Unfortunately, such dreams 
too often meet with more acceptance than the drier 
deductions from observed facts ; which must, how- 
ever, in the end, form the only basis of all geologic 
science. But there is no occasion here to urge even 
the most imaginative to resort to hypothesis, for the 
formation of columns in other substances than B. 
is quite familiar, and their producing causes evident. 
In starch, columns having the external prismatic 
appearance, and the internal earthy structure, are 
produced simply from the escape of vapour, and 
consequent shrinking of parts. We have seen 
singularly regular joints produced in the argillaceous 
ironstone at Wardie, near Edinburgh, on its expo- 
sure on the beach, the contractions forming the 
columns evidently resulting from the escape of the 
moisture retained by the bed while it was covered 
by other strata. The same occurs in beds of fine 
clay that have been recently exposed. But the 
most regular series of columns that have been 
noticed by us, were produced on bricks which 
formed the bottom of a public oven. The long- 
continued and powerful heat to which they had 
been subjected, though it had not caused fusio;ls, had 
1