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times refemble fine rays runniiig in
different directions, that do not cor-
refpond with the internal ilrnCtiire o f
the cryftals, which I have endeavoured
to examine in another place.
From what I have Intherto mentioned,
the opinion that the bafalts
have been produced by cryitalhzation,
becomes at leail lefs probable, whether
we admit the wet or dry method.
But I muil not omit that the fpars exhibit
a kind o f cryilallization, which
at firil fight refembles a heap of bafalts
; but, upon a clofer examination,
a very great difference is obferved.
The form of the fpar is every where
alike, but the bafalts differ from one
another in point of fize and number
o f fides ; the former, when broken,
coniiils o f many fmall unequal cubes,
but the bafalt does not feparate in
regular parts, &c.
Nature’s fecond method to produce
regular forms is that o f cruil-
iiig the outer furface of a melted
mafs. By a iudden refrigeration
nature, to effeCt this purpofe, makes
pfe o f polyedrous and irregular
5 forms.
fprmf.. I f wp fuppi^fe a coniiderable
bed, wliich.4s.,.^l:)ecome fluid by fire,
Fpfead. qver a .plain, it evidently
appears, that the furface muft firil o f
all lofeithe degree o f heat requifite for
melting,, and begin to congeal ; • but
the cold requifite for this purpofe like-
wife cpntracfy the uppermoft congeal-
f d ‘ ftratum into a narrower ipace,
and confequently caufes it to feparate
from the remaining liquid mafs,, as
the iide expofed to the air is already
too ftiff.to give wiiy» In this manner
a ftratum is produced running in a
parallel diredion with the whole mafs,
others ftill are produced by the fame
caufe, in px'oportion as the refrigeration
penetrates deeper.
Hence we may, in my opinion, very
plainly fee how a bed may be divided
into ftrata. In the fame manner the
refrigeration advances on the fides,
and confequently divides the ftrata
pieces of pillars, that
can hardly ever be exadly fquare,
as the ftrongeft refrigeration into the
inner parts o f the mafs advances ab
moft in a diagonal line from the corfl
b neiT,
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