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730 VON TROIL’S LETTERS ‘ON ICELAND.
fhould be free enough to fix themfelves in a certain order; and as this is not pratti-
cable in a large melted mafs, no cryftallizations appear in it, except on its furface, or
in its cavities.
Add to this, that the bafalts in a frefh frature, do not fhew a plain fmooth furface
under the microfcope, but appear fometimes like grains of a different magnitude, and
at other times refemble fine rays running in different directions, which does not cor-
refpond with the internal ftru€ture of the cryftals, which I have endeavoured to examine
in another place.
From what J have hitherto mentioned, the opinion that the bafalts have been pro-
duced by cryftallization, becomes at leaft lefs probable, whether we admit the wet or
dry method. But I muft not omit that the fpars exhibit a kind of cryftallization,
which, at firft fight, refembles a heap of bafalts, but, upon a clofe examination, a very
great difference is obferved. ‘The form of the fpar is every where alike, but the ba-
falts differ from one another in point of fize and number of fides; the former, when
broken, confifts of many fall unequal cubes, but the bafalt does not feparate in regular
parts, &c. &c.
Nature’s fecond method to produce regular forms is that of crufting tae outer fur-
face of a melted mafs. By a fudden refrigeration Nature, to effect this purpofe, makes
ufe of polyedrous and irregular forms. If we fuppofe a confiderable bed, which is be-
come fluid by fire, and fpread over a plain, it evidently appears that the furface muft
firft of all lofe the degree of heat requifite for melting, and begin to congeal ; but the
cold requifite for this purpofe likewife contracts the uppermoft congealed ftratum into
anarrower fpace, and confequently caufes it to feparate from the remaining liquid
mats, as the fide expofed to the air is already too {tiff to give way. In this manner a
ftratum is produced running in a parallel direction with the whole mafs, others ftill are
produced by the fame caufe, in proportion as the refrigeration penetrates deeper.
Hence we may, in my opinion, very plainly fee how a bed may be divided into ftrata.
In the fame manner the refrigeration advances on the fides, which confequently divides
the ftrata into polyedrous pieces of pillars, which can hardly ever be exaétly fquare, as
the ftrongeft refrigeration into the inner parts of the mafs advances almoft in a diagonal
line from the corners. If we add to this, that a large mafs cannot be equal throughout
its compofition, nor every where liquid in the fame degree, it will be eafy to difcover the
caufe of feveral irregularities. Hf the depth of the bed is very confiderable, in proportion
to its breadth, prifmatic pillars, without crofs divifions, are produced, at leaft length.
ways, from the uppermott furface downwards.
The third way is perfectly fimilar to the preceding in refpect to the effeét, but is diffe-
rent from it by the mafs being foaked with water, and by the burfting of it afunder,
being the effeét of the contraction whilft it is drying. If we fuppofe fuch a bed to be
fpread over a level fpace, the drying advances in the fame manner as the refrigeration
in the former cafe.
This feparation into ftrata properly happens when a confiderable quantity of clay enters
into the whole compofition, becaufe the clay decreafes more than any other kind of earth
in drying.
We a now examine which of thefe two ways may beft ferve to explain the manner
in which the bafalts are produced, for it is hardly poffible that they fhould have been
formed by cryftallization. ;
However well founded the opinion may appear of deducing them froma melted fub-
ftance, feveral very confiderable objections however may be raifed againft it, which I
fhall not forget to mention. It feems therefore more credible to me, that they have been
13 produced
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