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VON TROIL’S LETTERS ON ICELAND. 725
places are grains refembling quarz, which are eafily diflolved in fufible urinous falt, but
do not fhew the leaft effervefcence in fal fodz.
21. The {mall cryftals, faid to refemble cocks-combs ; they are nothing but a different
appearance or change of the heavy fpar.
22. Achalcedon cruft with fmooth prominencies, like what they call hamatites : this
is diffolved with the utmoft difficulty by fufible urinous falt, with more eafe by borax,
and with a violent effervefcence by fal fodz, exactly as the flint.
23. Zeolite; two kinds: the one is folid, white, and internally, as it were, compofed
of globofe parts, in which rays proceeding from the center appear that refemble fine
threads.
This fort fwells a little by the blow-pipe, diflolves perfeétly with borax, feparates in
fal fodz with fome effervefcence, but foon ceafes, and leaves fome part undiffolved.
The other fort confifts of a plate, which in colour and break refembles a carnelian : it
hasa quantity of {mall prominencies in it, filled with irregular white cryltals, and fome
of the cavities are filled with a loofe-grained and brownifh-red fubftance.
The fubftance refembling a carnelian becomes white in the fire, bubbles up, and be-
comes fufible.
The cryftalline fubftance becomes more frothy in the fire than the carnelian, and has
all the qualities of the zeolite.
The fandy fubftance hardly fwells; is diffolved with difficulty by borax, and is ate
tacked at firft with a fudden effervefcence by fal fod.
As it is not uncommon even in the profeflors of morality to pafs from one wrong f{tep
to another, fo are we not without examples of this kind in thofe who make nature their
ftudy. ‘Ten years ago it was a general opinion that the furface of the earth, together
with the mountains upon it, had been produced by moilture. It is true, {ome declared
the fire to be the firft original caufe, but the greater number paid little attention to this
opinion. Now, on the contrary, that a fubterraneous fire had been the principal agent
gains ground daily : every thing is fuppofed to have been melted even to the granite.
My own fentiments with regard to it is this, that both the fire and water have contri-
buted their fhare in this operation, though in fuch a proportion, that the force of the
former extends much further than the latter; and, on the contrary, that the fire has
only worked in fome parts of the furface of the earth.
It is not an eafy matter to explain how the granite, which confifts of clear quarz-
eryftals, folid field-fpar, and glimmer (mica) with flat feales, has been able to fupport a
fufion, without the quarz burfting, or becoming opaque.
This is yet lefs to be conceived of the field-fpar, that becomes foft and liquid in a
weak fire, and has a dull appearance. ‘lhe glimmer fplits its fcales afunder in the fire,
and frequently twits them together again in a very different manner from that in which
they appear in the granite. Notwith{tanding all this, if the granite is confidered as a
production of the fire, it need not be wondered at that the zeolite has likewife been com-
prehended in this fuppofition.
I will allow that cryftals may be produced by the dry method, and I know feveral
ways of obtaining them, both by fufion and fublimation ; but I can never be perfuaded
that the zeolite has been produced by the affiftance of fire. It is true, that fometimes
they are found in loofe ftones, and in fuch places where volcanos had formerly raged’:
it is likewife found in folid rocks that have never been expofed to thefe fires, as at
Guftavenberg in Jemtland.
If more forts than one are alfo certainly free from all fufpicion of having been fabject
to fufion, how is it poflible, without the cleareft proofs, to {uppofe that the whole genus
has
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