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724 VON TROIL’S LETTERS ON ICELAND.
The white, which poffeffes more or lefs of thofe tranfparent grains or rays with which
the lavais chequered, do not feem to be of the nature of quarz, as they cannot be at-
tacked by fal fod; they are however, with fome difficulty, diflolved by borax and
fufible urinous falt.
Thefe effets are perfectly fimilar to thofe produced upon the diamond, ruby, faphire,
topaz, and hyacinth. ‘The chryfolite, garnet, turmaline, and fhirl, can neither be dif-
folved by fal fodee, though they are fomewhat attacked by it, when reduced to a fine
powder ; and upon the two laft mentioned ones it produces a flight effervefcence. On
this account it is poflible that the precious {tones of mount Vefuvius, which are fold at
Naples, are nearer related to the real precious {tones than is generally imagined.
ig. A finer kind of lava, quite porous within, and entirely burnt out, and confiderably
lighter on that account than the preceding ones. I have not found any fuch grains im
it refembling quarz.
16. The fo called Icelandic agate. his is of a black or blackifh-brown colour, and
a little tranfparent at the thin edges like glafs, and gives fire with the fteel.
It cannot eafily be melted by itfelf, but becomes white, and flies in pieces. It can
hardly be diffolved in the fire by fufible urinous falt ; but it fucceeds a little better with
borax, though with fome difficulty : with fal fodze it diflolves very little, though in the
firft moments fome ebullition is perceived, and the whole mafs is afterwards reduced to
owder.
s From hence it may be concluded, that the Icelandic agate has been produced by an
exceflive fire out of the lava defcribed in N° 14. I have found no cryftals of this glafs
in the collection. If any perfon has fuch in their pofleflion, they fhould be examined
to fee if they are exactly of the fame nature and fubftance with the above defcribed agate,
and if their form has not been produced accidentally by burlting afunder.
17. More or lefs light, fpongy, and burnt-out pumice {tone, particularly black and
reddifh-brown. © Quarz cryftals are fometimes found in them; but oftener the rays and
grains refembling quarz.
Stones thrown out of the volcano, grey or burnt brown, which feem to confift of a
hardened clay mixed with filicecus earth. ‘They are {prinkled with rays and grains re-
fembling quarz, and fome few flakes of mica. They fufe with great difficulty in the
fire ; with fal fodz they fhew fome effervefcence at firft, but however it foon ceafes : the
parts refembling quarz do not produce any motion atall. From this we may conclude,
that the lava mentioned in N° 14, principally originates from this mafs.
The other loofe ftones which I have received from you, Sir, to all appearance have
no abfolute connection with the eruptions of fire, though fome have been fufpected of
it. I will enumerate them kere feparately.
19. Red and green jafper, which, in fome places where it is broken, is quite fmooth
and fhining ; this circumltance diftinguifhes it from the common jafper, which is dull
and clay-like where it is broken. It has befides all the qualities of true jafper; ftrikes
fire with the f{teel, does not melt in the moft violent fire, but is diffolved by fal fodze
with an effervefcence, &c. &c. As to the fmoothnefs of fome parts, it cannot proceed
from a commencing fufion, as it becomes black and dark in a weaker fire. We here
only find a new link in the connected chain of nature, by which the jafper is united
with the flint.
There is no black jafper in your colle€tion; but the pieces, which to appearance
comes neareft to it, belong to the clafs of the trapp, and fhall be defcribed hereafter.
20. Grey, greenifh flate, refembling jafper, that gives {parks with the fteel, is at-
tacked with a quick effervefcence by fal fode, but not farther diffolved by it. In fome
places
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