niformly mixed, but every where separating and
subsiding from the pure saline substance.

There is also to be observed a certain regularity in this separation of
the tinging from the colourless substance, which, at a proper distance,
gives to the perpendicular section of the rock a distinguishable figure
in its structure. When looking at this appearance near the bottom of
the rock, it, at first, presented me with the figure of regular
stratification; but, upon examining the whole mass of rock, I found,
that it was only towards the bottom that this stratified appearance took
place; and that, at the top of the rock, the most beautiful and regular
figure was to be observed; but a figure the most opposite to that of
stratification. It was all composed of concentric circles; and these
appeared to be the section of a mass, composed altogether of concentric
spheres, like those beautiful systems of configuration which agates so
frequently present us with in miniature. In about eight or ten feet from
the top, the circles growing large, were blended together, and gradually
lost their regular appearance, until, at a greater depth, they again
appeared in resemblance of a stratification.

This regular arrangement of the floating marly substance in the body
of salt, which is that of the structure of a coated pebble, or that
of concentric spheres, is altogether inexplicable upon any other
supposition, than the perfect fluidity or fusion of the salt, and the
attractions and repulsions of the contained substances. It is in vain
to look, in the operations of solution and evaporation, for that which
nothing but perfect fluidity or fusion can explain.

This example of a mineral salt congealed from a melted state, may be
confirmed from another which I have from Dr Black, who suggested it to
me. It is an alkaline salt, found in a mineral state, and described in
the Philosophical Transactions, _anno_ 1771. But to understand this
specimen, something must be premised with regard to the nature of fossil
alkali.

The fossil alkali crystallises from a dissolved state, in combining
itself with a large portion of the water, in the manner of alum; and,
in this case, the water is essential to the constitution of that
transparent crystalline body; for, upon the evaporation of the water,
the transparent salt loses its solidity, and becomes a white powder. If,
instead of being gently dried, the crystalline salt is suddenly exposed
to a sufficient deg