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732 VON TROIL’s LETTERS ON ICELAND.
From what I have hitherto faid, you will perceive it is my opinion, that the bafalts
have been produced by the affiftance of a fubterraneous fire, but that it is not yet de-
termined whether they have been rent afunder after the fufion, or by drying: this
laft however appears more credible to me on account of the reafons I have mentioned.
For to fpeak ftrictly, the fubftances inclofed in the bafalts, though they fhould even be
volcanic, do not yet with certainty prove a preceding fufion, as a fubftance foftened by
water may be as proper for it as one fufed by fire. {Iam however very far from being
inclined to maintain my opinion any farther than it agrees with certain experiments
and experience.
Truth will fooner or later be difcovered ; and I know nothing more derogatory to
the honour of a natural hiftorian, than having wilfully obftructed its paflage.
I will now give a more full defcription of the bafalts and different kinds of trapp
which you have brought from Staffa and Iceland.
24. Bafalt from Staffan—The piece prefented to me is a prifmatic hexagonal frag-
ment, three fides of which are almoft of equal dimenfions, and are connected with one
another ; two others are larger, and are feparated from one another by the fixth and
{malleft; it is a little concave at the top, and convex at the bottom.
Trapp is generally found in fquare irregular cubes, whence it has moft probably
obtained its denomination, on account of fome fimilarity with ftones made ule of for
ftair-cafes.
It is alfo found in prifmatic triangular forms, though rarely, as alfo in the form of
immenfe pillars. Of this kind are thofe called Traeleftenar, oppofite Bragnum, at the
foot of the Hanneberg, which have feparated themfelves from the remaining part of
the bed ; and in 1759, when I firft faw them, formed an angle of about eight degrees
with the plumb-line.
The bafalt from Staffa, when newly broken, is of a blackifh grey, fhining, and {mall
fealed; and I have difcovered with the microfcope fome fmall white particles fprinkled
up and down.
The fineft trapp is perfectly fimilar when broken, only of a lighter colour, which
proceeds from the greater quantity of white particles.
The furface decays to a grey yellow loofe cruft, which lofes itfelf in the more folid
mafs.
The fine trapp decays in the fame manner.
The bafalt when {truck with the fteel hardly gives fire, though a fpark may now and
then be obtained with difficulty.
This fame circum{tance may be obferved of the trapp.
Its {pecific weight is about 3000, and that of the trapp about 2990.
It becomes very beautiful by cutting, polifhing, and grinding.
Likewife the more fine kind of trapp.
It yields an afh-coloured powder.
The trapp yields rather a powder of a more light colour.
It foon melts to black glafly flags.
The trapp likewife.
The bafalt is attacked by fodz with an effervefcence which foon ceafes, and, though
fome feparation enfues, the greater part however remains undiffolved. Borax per-
feCtly folves it without effervefcence, and gives a clear iron-coloured glafs. It is folved
with great difliculty by fufible urinous falt, and whilft cooling becomes of a whitifh grey,
and not tran{parent.
The fame effects are produced by thefe acids upon the trapp.
4 One
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