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VON TROIL’S LETTERS ON ICELAND. 733
One hundred parts of bafalt very finely powdered, and feveral times digefted with
frefh aqua regia, and then well wafhed and dried, leave fixty-eight parts undiflolved.
The remainder of this fhews a little effervefcence before it unites with the fal fodz,
and diflolves very little. It is diffolved with eafe by borax, and with difficulty by fufible
urinous falt. It feems therefore to be a mixture of filiceous and gemmeous earth.
The folution gives by precipitation with lixivium fanguinis as much Pruffian blue as
is equal to twenty-fix parts in one hundred of iron ; though the bafalt, by being tried
in the ufual manner in the crucible, does not yield above ten in one hundred. ‘This
proves that lixivium fanguinis affords the moft exact method of affaying iron ore.
When at laft the folution is precipitated with cauftic volatile alkali, after the iron has
been feparated by lixivium fanguinis, faturated with acid, pure argillaceous earth is
obtained.
Sometimes a little calcareous earth appears after a preceding precipitation; when
diffolved fal fodze is added; but fometimes not the fmalleft traces of it can be difcover-
ed, even with the acid of fugar, which is however the fafeft method hitherto known of
difcovering it. The calcareous earth feems therefore to be accidental. ‘Vhis is how-
ever very certain, that the interftices between the pillars are fometimes found filled up
with calcareous {par.
The trapp is exa€tly of the fame nature, and contains nearly the fame allay, fo that
the experiments differ only one or a half part in one hundred. ‘The moft cofiderable
difference confifts in the calcareous earth appearing here more wifibly, fo that generally
a flight effervefcence is obferved when an acid is poured on the powder.
25. Bafalt from Hvitara, near Skalholt, in Iceland.—The piece in my poffeflion is
too {mall to difcover its form; only a part of the outfide can be diftinguifhed. When
frefh broken it refembles the bafalt from Staffa, though fomething may be obferved in
it which is very feldom difcovered in the laft. Thefe are {mall round cavities, not
larger than pins’ heads, thinly fcattered in fome places, as likewife on the outfide. All
thefe cavities are filled up with a white, green, or brown powder.
May not thefe perhaps have been fome particles of a fubftance, which eafily diffoly-
ing me become liquid, though the whole mafs had not a fufficient degree of heat to
melt it !
But whence can thefe cavities be filled with this powder ? In the midft of fo folid a
maf{s, no decay feems to be poifible.
The fubftance of the bafalt itfelf produces a little effervefcence with fal fode, and
feparates without being vifibly diffolved. It diffolves in borax, as likewife in fufible
urinous falt, aithough with more difficulty. By the common method of proving it in
the crucible it yields ten parts in one hundred of iron. ‘he fame circumf{tances may
be obferved in trying the powder that fills up the cavities; it only feems to melt a
little eafier than the folid fubftance furrounding it.
26. The bafalt from Langarnas perfectly relembles coarfe trapp, though it has more
white particles, and fometimes cryftallizations as large as a cherry-ftone ; the dark grey
and white parts prove to be entirely of the fame nature by the blow-pipe, becomes fluid
by itfelf, and produces a fudden effervefcence with fal foda, but without being quite
diffolved by it: it is exaétly as the preceding fort.
27. Black, folid, glofly trapp, knotty within, and refembling wood in its internal
ftructure, being full of filaments. Many pieces are grown to acruft of pumice on one
fide or another, though their edges are quite frefh and fmooth; whence we may con-
elude, that they themfelves have not been melted, but have either been thrown into the
lava,
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