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be obferved for 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 Laugarnas per-
feftly refembles 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 o f the fame
nature by the blow-pipe, becomes
fluid by itfelf, and produces a fudden
effervefcence with fal fodæ, but without
being quite diffolved by it: it is
exadly as the preceding fort.
27. Black, folid gloffy trapp, knotty
within, and refembling wood in its
internal ftrudure, being full o f fibres.
Many pieces are grown to a cruft o f
pumice on one iide or another, though
their edges are quite frefli and fmooth;
whence we may conclude, that they
themfelves have not been melted, but
have either been thrown into the
lava, which was already burnt out,
or that the lava has flowed over
them ; fome part o f it however feems
to
to have been more attacked by the
fire.
The manner in which the fire and
acids operate upon this trapp, is exa
d ly the fame as with the preceding
bafalt. There are likewife fome grains
fcattered in it refembling quarz, which
are not folved by fal iodas, nor does
it caufe any efferveicence; borax and
fufible urinous fait entirely diffolve
them, though flowly.
28. A compad dark-brown trapp
from Vido, the furface o f which is
glaify and uneven, as i f it had been
made fluid by fire. It muft alfo be
obferved, that cryftallizations o f frefli
pyrites are frequently found in thefe
glaffy rough pieces. The glaify fubftance
eafily becomes fluid with fome
ebullition, almoft like fliirl ; befides,
it exhibits the fame circumftances with
acids as the preceding.
You will fee, Sir, by this long letter,
that in the eruptions in Iceland
argillaceous and iiliceous fubftances
have been principally concerned, as
has been the cafe in other parts. I
know very well that Mr. Beaume
niain