'■ il
■Bf
n
A,:
!A kU '7"
k fi Bt
The bafalt mountains feem to be
very antient, at lead I do not know
that the age o f any one is afcertained.
Should they then be fo old, that tlie
fubftance o f the trapp was not yet
perfedly hardened, when were they
produced ? Beildes, we frequently
find to this day clayey fubftances at
a great depth, which are fo foft that
they may be fcraped by the nail, but
afterwards become very hard when
expofed to the air.
There have without doubt been
many eruptions o f fire on the ifle o f
Staffa, as the fituation of the pillars
and their being removed out of their
places evidently prove.
You, Sir, have likewife brought a
very clear proof o f this from thence,
which is a piece o f bafalt, that ex.
teriorly is full o f hollows, and in a
manner burnt.
A hard fubftance, when expofed to
a degree o f heat infufiicient to melt
the whole piece, may however be attacked
by it in fome parts o f the
furface moft liable to become fluid.
The
The mixture o f a large mafs is feldom
every where fo uniform, that fome
parts fliould not be more liable to melt
than others. ,
Crooked pillars may be produced
as well by the drying as the refrigeration
o f a liquid mafs; for this purpofe
it is only neceffary that the furface
Iliould be bent, as the ftratum always
runs ill a parallel direkfion with it.
From what I have hitherto faid you
wiii perceive it is my opinion, that
the bafalts have been produced by the
affiftaiice o f a fubterraiiean fire, but
that it is not yet determined whether
they have been feparated by the
fufion, or by dryiiigi this laft however
appears more credible to me on
account o f the reafons I have mentioned.
For to fpeak ftricftly, the
fubftances inclofed in the bafalts,
though they fliould even be volcanic,
do not yet with certainty prove a
preceding fufion, as a fubftance fof-
tened by water may be as proper for
it as one fufed by fire. I am however
very far from being inclined to
B b 4 main-
Ii'/