A
rude and yet helpful conception can be gained of it by considering how
the soft dough, under heat, the expansion of included gas, and
prolonged standing in these conditions, becomes the more or less hard
and brittle bread. The soft clay put in the furnace, shaped into the
form of various utensils, is similarly metamorphosed into the ringing,
dense, stone-like ware. Such illustrations fall very far short of
accuracy, and yet they impose upon the mind at least this idea of a
physical hardening, in which new densities and new chemical
combinations, or assortments of combinations, take place in the
hardening body. In fact, in nature it is most likely that in all
instances of metamorphism there has been a mineralizing process going
on, in which heated waters have penetrated the hardening, crystallizing
menstruum at the beginning, at the middle, or even at the end of the
metamorphic action, and have not only assisted the mobility of the mass
of sediments, but have brought to it new elements.
Such
metamorphic action can be greatly varied. It can be, as regards time,
slow or more rapid; as regards agencies, it can arise from earth
movements or from the proximity and intrusion of lavas rising from
deep-seated sources ; as regards phase, it may be complete or
incomplete, the former indicating a condition of more mineral
complexity or alteration.
All
sedimentary deposits are more or less filled, in the interstices, with
water, which as a universal solvent is never free from dissolved
substances. This water heated becomes active in its solvent powers,
and, as the beds are compressed, lifted and hardened this water begins
a mineralizing influence throughout. Pressure and heating continue, and
while not probably at high temperatures, at least, at first, a rapid
mineralization goes on. The elements sort out into compounds, as
silicates, of which those most readily formed are the most numerous. A
semi-fusion succeeds and, the pressure continuing, a schistose
structure is developed, the longer axes of the flakes and crystals
lying together along the planes of the rock cleavagei. Such beds
exhibit plication and folding, and
