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690 VON TROIL’S LETTERS ON ICELAND.
{tones, which are fometimes thrown to an incredible diftanee. I have feen a round {tone
near Nafeirholt, about a mile from Heckla, which was an ell in diameter, and had been
arown there in the laft eruption of Heckla. Egbert Olafsen alfo relates, that at the
Jaft eruption of Kattlegiaa, a {tone which weighed two hundred and ninety pounds was
thrown to the diftance of four miles.
A quantity of white pumice-{tone is alfo thrown up with the boiling waters ; and it is
elieved, with great probability, that the latter proceeds from the fea, as a quantity of
{alt fuflicient to load feveral horfes has frequently been found after the mountain has dif-
continued burning. .
Then follows venerally brown or black pumice-ftone, and lava, with fand and afhes.
The lava is feldom found near the opening, but rather ¢wffa, or loole afhes and grit;
and indeed the greater part of the Icelandic mountains confifts of this matter, which,
when it is grown cold, generally takes an arched form, fome admirable proofs of which
may be feen in the cleft at Allmanagiaa: the upper cruft frequently grows hard and
folid, whilft the melted matter beneath it continues liquid; this forms great cavities,
whofe walls, bed, and roof are of lava, and where great quantities of ftalactite of lava
are found.
There are a great number of thefe caves in Iceland, fome of which are very large,
and are made ule of by the inhabitants for fheltering their cattle. I will here only take
notice of the cave of Surtheller, as the largeft of all: it is between thirty-four and thirty-
fix feet in height ; its breadth is from fifty to fifty-four feet, and it is five thoufand and
thirty-four feet long.
It would be both tedious and difficult to clafs the different compofitions of fire in
thefe places, as it is not eafily difcovered to which they belong : for example, jafper, of
which great quantities of red and black are found inclofed in the lava, and mixed with
it; I will therefore only name thofe which have been evidently produced by the fire.
Firft, ¢vffa, a ftone, feruminated afhes and grit, which fometimes is found mixed with
lava, bafalt, and other forts of ftones, and having been moiftened by the {pouting of
water, grows hard by heat and length of time. Secondly, /ava, is that kind of ftone
which has been melted by the violence of the fire, and varies according to the difference
of the {tate in which it ferved as food to the fire. This lava is fometimes found folid,
and at others porous and full of bladders and holes; in the infide it is filled up with
opaque and brittle fquare cryftals of a dead white, or with green drops of glafs, which
decay after they have been long expofed to the air. The colour of the lava is black,
dark blue, purple, reddifh brown, or yellowifh, but ofteneft black or red. Where the
fire has operated very {trongly, it is, as it were, glazed, and looks like refin. In the
frames or great tracts of lava it is fometimes obferved, that the cruft in growing cold
has laid itfelf into folds; but generally it forms itfelf into a refemblance of a rope or
cable, fometimes lengthways, and at others in the form of a circle, like unto a great
cable rolled together ; and generally fo, that its thicknefs continually augments from
the centre to the periphery. ‘To this clafs | muft alfo count a black folid matter, which
(trikes fire again{t fteel, and-fometimes takes the forms of trees or branches: fome
people have been inclined to think they are petrified trees, but I am rather of opinion
that it is a real jafper. ‘Thirdly, pumice, black, red, and even white, which lat has
molt probably been difcoloured by the boiling water. Fourthly, agate; I preferve the
received name, though it is really nothing more than burned glafs. In fome few places
it is found white, tran{parent, and almoft in the form of cryftal. The bluith fort is alfo
rare, but found in large pieces: the moft common is the black agate, which is found
generally in itratas, or in {mall nefts, and fometimes ar in the fhape of cryftal, in
oval,
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