there
must have been a sensible plasticity permitting the innumerable major
and minor flexures. Should the beds become rigid, the folding process
results in faults and fractures. Shearing or long strains and movement
along certain planes aid mineral changes, developing heat, and thereby
causing secondary solution and redeposition. The mica schists result
from clay beds infused with potash or iron waters, with magnesia
salts, such as might readily remain in beds laid down in sea-water.
Silica, so universally distributed in gneissoid rocks, has been brought
up in heated waters or separated out from the original mass of
sediment, where it may have been a sand, as a crystalline unit.
There
is both local, or contact, and regional metamorphism, the former
occurring around intrusive dikes and bosses, as in Norway, in the Harz,
in Scotland, in New England, and the latter, extendedly developed over
a whole area, subjected to secular crustal motions and shortenings, as
on our island. As a true metamorphic instance the conversion of a soft
coal seam into natural coke by an intruded rock may be alluded to. The
metamorphism of calcareous muds, making of them marbles, is practically
a baking, a change of structure, of density, etc., with a probable
accompaniment of developing silicates, as tremolite, tourmaline,
phlogopite, diopside, etc.
Progressive
metamorphism denotes a consecutive increase of crystalline structure
and contents, as when clays baked into slates develop, under further
metamorphic stress, chiastolite, quartz, mica.
Besides
the metamorphism of sediments, there are so-called metamorphic changes
in igneous lava-like bodies which come up through crevices in the
earth's surface, and are essentially natural slags or glasses. Their
metamorphism consists—after their consolidation and crystallization—in
mineral changes, whereby also under pressure, shearing, etc., and by
mineralizing agents as water and fluorine, new minerals develop or old
ones enlarge, and the unstratified magma becomes foliated, zoned and
schistose. It is an inverted metamorphism.
