(Delesse Ann. des Mines 1852 tome 3 page 409 and 1848 tome 13 page
675.)
As a general rule, quartz, in a compact or amorphous state, forms a vitreous
mass, serving as the base in which feldspar and mica have crystallised; for
although these minerals are much more fusible than silex, they have often
imprinted their shapes upon the quartz. This fact, apparently so paradoxical,
has given rise to much ingenious speculation. We should naturally have
anticipated that, during the cooling of the mass, the flinty portion would be
the first to consolidate; and that the different varieties of feldspar, as well
as garnets and tourmalines, being more easily liquefied by heat, would be the
last. Precisely the reverse has taken place in the passage of most granite
aggregates from a fluid to a solid state, crystals of the more fusible minerals
being found enveloped in hard, transparent, glassy quartz, which has often taken
very faithful casts of each, so as to preserve even the microscopically minute
striations on the surface of prisms of tourmaline. Various explanations of this
phenomenon have been proposed by MM.
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