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t 386 1
ners. I f we add to tHis, that a large
mafs cannot be equal throughout its
compoiition, nor every where liquid
in the fame degree, it will be eaiy to
difcover the caufe o f feveral irregularities,
I f the depth o f the bed is
very coniiderable, in proportion to
its breadth, prifmatic pillars, without
crols*diviiions, are produced, at leaft
lengthways from the uppermoft fuf-
face downwards.
The third way is perfedly fimilar
to the preceding in refped to the ef-
fed, but is different from it by the
mafs being foaked with water, and
by the burfting o f it afunder, which
is the effed o f the contradion whilft
it is drying. I f we fuppofe fuch q
bed to be fpread over a level fpace,
the drying advances in the fame manner
as the refrigeration in the former
cafe,
This reparation into ftrata properly
happens when a confiderable quantity
o f clay enters into the whole compoiition,
becaufe the clay decreafes
more than any other kind o f earth in
drying.
We
[ 387 ]
We muft now examine which o f
thefe two ways may beft ferve to explain
the manner in which the bafalts
are produced, for it is hardly pofiible
that they fliould have been formed by
cryftallization.
However well founded the opinion
may appear o f deducing them from a
melted fubftance, feveral very coniiderable
objedions may neverthelefs
be raifed againft it, that I fliall not
forget to mention. It feems therefore
more credible to me, that they have
been produced out o f their fubftance
whilft it was yet foft, or at leaft not
too hard to be fofteiied by exhalations.
I f we therefore fuppofe that
a bed is fpread over a place where a
volcano begins to work, it is evident
that a great quantity o f the water,
always prefent on thefe occafions,
is driven upwards in exhalations or
vapours ; thefe it is well known pof-
fefs a penetrating foftening power,
by means o f which they alfo produce
their firft effed; but when they are
increafed to a fufficient quantity, they
force this tough moift fubftance up-
B b 2 wards,
fo