Bergman has likewise shewn, that the limestones which become bluish or
dark coloured when calcined, possess a mixture of manganese, and are
thence preferable as a cement to other kinds of lime. 2. Bergman, 229.
Which impregnation with manganese has probably been received from the
decomposition of superincumbent vegetable matters.

These cracks or perpendicular caverns in the granite or limestone pass
to unknown depths; and it is up these channels that I have endeavoured
to shew that the steam rises which becomes afterwards condensed and
produces the warm springs of this island, and other parts of the world.
(See note on Fucus, Vol. II.) And up these cracks I suppose certain
vapours arise, which either alone, or by meeting with something
descending into them from above, have produced most of the metals; and
several of the materials in which they are bedded. Thus the ponderous
earth, Barytes, of Derbyshire, is found in these cracks, and is
stratified frequently with lead-ore, and frequently surrounds it. This
ponderous earth has been found by Dr. Hoepfner in a granite in
Switzerland, and may have thus been sublimed from immense depths by
great heat, and have obtained its carbonic or vitriolic acid from above.
Annales de Chimie. There is also reason to conclude that something from
above is necessary to the formation of many of the metals: at Hawkstone
in Shropshire, the seat of Sir Richard Hill, there is an elevated rock
of siliceous sand which is coloured green with copper in many places
high in the air; and I have in my possession a specimen of lead formed
in the cavity of an iron nodule, and another of lead amid spar from a
crack of a coal-stratum; all which countenance the modern production of
those metals from descending materials. To which should be added, that
the highest mountains of granite, which have therefore probably never
been covered with marine productions on account of their early
elevation, nor with vegetable or animal matters on account of their