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Sometimes a little calcareous earth
appèars after a preceding precipitation;
when dhTolvcd fal fodæ is added;
but fometimes not the fmaileil traces
o f it can be difcovered, even with the
acid o f fugar, which is however the
fafeft method hitherto known o f dife
covering it. The calcareous earth
feems therefore to be accidental. This
is however very certain, that the interftices
between the pillars are fometimes
found filled up with calcareous
fpar.
The trapp is exacfly o f 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 confiderable differ«“
ence coniifts in the calcareous earth
appearing here more viiibly, fo that
generally a flight effervefcence is obferved
when an acid is poured on the
powder.
25. Bafalt from Hvitara, near Skallholt,
in Iceland.— The piece in my
poiTeffion is too fmall tô difcover its
form ; only a part o f the outfide can
be diftinguiihed. When freili broken
5 it
it refembles. .the bafalt from Staffa,
though fomething may be obferved
in it which is very feldom difcovered iu
the laft. Thefe are fmall 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,
greenifli, or brown powder.
May not thefe perhaps have been
fome particles o f a fubftance, which
eafily diifolving was become liquid,
though the whole mafs had not a fufiicient
degree o f heat to melt it ?
But whence can thefe cavities be
filled with this powder ? In the midft
o f fo folid a mafs, no decompofition
feems to be poffible.
The fubftance o f the bafalt itfelf
produces a little eifervefceuce with fal
fodæ, and feparates without being vifi-
bly diffolved. It diifolves in borax, as
likewife in fufibleurinousftlt, although
with more difficulty. By the common
nfythod o f proving it in the crucible it
yields ten parts in one hundred o f
iron. T ne fame circumftances may
be
fo