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VON TROIL’S LETTERS ON ICELAND. at
produced out of their fubftance whilft it was yet foft, or at leaft not too hard to be
foftened by exhalations. If we therefore fuppofe that a bed is fpread over a place where
a volcano begins to work, it is evident that a great quantity of the water always prefent
on thefe occafions, is driven upwards in exhalations or vapours ; thefe, it is well known,
poflefs a penetrating foftening power, by means of which they alfo produce their firft
effect ; but when they are increafed to a fufficient quantity, they force this tough moift
fubftance upwards, which then gradually falls, and during this time burlts in the man-
ner defcribed above.
’ My reafons for this opinion are thefe; firft, we do not find the internal grain of the
bafalts melted or vitrified, which however foon happens by fufion, and for which pur-
pofe only a very fmall degree of fire is requifite. It confequently is very hard to ex-
plain how this fubftance could have been fo fluid, that no traces of bubbles appear in it (at
leaft I have not been able to difcover any, after the niceft examination into the Scotch
and Icelandic bafalts) and yet when broken appear dull and uneven. I know very
well that lava is feldom vitrified within ; but the great number of bubbles and pores
which are found in the whole mafs, are more than fufficient proofs that it has not
been perfeétly melted to its f{malleft parts, but has only been brought to be near
fluid.
Secondly, the bafalts fo much refemble the finer trapp, both in refpeé to their
grain and original compofition, that they can hardly be diftinguifhed in {mall frag-
ments, as will be more plainly proved in the comparifon which I will make hereafter.
See N® 24. :
But the trapp in all probability has never been melted, at leaft not in thofe parts
where I have had opportunities of examining it.
Almoft in all the weft Gothic ftratified mountains, the uppermoft ftratum is trapp ;
and it muft be well obferved, that it always lies wpon black allum flate. Is it there.
fore credible that this fubftance, which in many places extend above a hundred yards,
can have been perfectly melted, without caufing the flate lying beneath it to lofe fome
part of its blacknefs, even in thofe places where they touch one another, as this effect
may be produced in a {mall culinary fire ? 2
There is befides a finer kind of trapp, which is generally found in veins or loads,
and frequently in very ancient mountains, where not the leaft traces of {ubterraneous
fire are to be feen.
The bafalt mountains feem to be very ancient, at leaft { do not know that the age
of any one is afcertained. Should they then be fo old, that the fubftance of the trapp
was not yet perfectly hardened, when were they produced? Befides, we frequently
find to this day clayey fubftances at a great depth, which are fo foft that they may be
{craped by the nail, but afterwards become very hard when expofed to the air.
There have without doubt been many eruptions of fire on the ifle of 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 of this from thence, which is a
piece of bafalt, that on the exteriorly is full of hollows, and in a manner burnt.
A hard fubftance, when expofed to a degree of heat infufficient to melt the whole
piece, may, however, be attacked by it in fome parts of the furface molt liable to be-
come fluid. The mixture of a large mats is feldom every where fo uniform, that fome
parts fhould not be more liable to melt than others.
Crooked pillars may be produced as well by the drying as the refrigeration of a
liquid mafs; for this purpofe it is only neceffary that the furface fhould be bent, as the
stratum always runs in a parallel direction with it.
5A2 From
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