
ALTERATION AND METAMORPHISM 221 
aqueous sediments which had been gradually deposited upon the floor 
of the ocean. When a great thickness of strata had accumulated, the 
loose sediments were supposed to have been consolidated by the pressure 
of the overlying masses. The internal heat of the earth next began to 
soften the compressed strata, and even eventually to melt them. The 
melted portions were thought to be now represented by granite, etc., 
while the strata which were only softened by the “internal fire,” now 
formed our crystalline schists. The view held by Hutton and his followers 
still finds many supporters. But with our increased knowledge of the 
geological structure of the earth’s crust, and of the chemical and physical 
conditions which have played their part in modifying rocks, it is needless 
to say that the views of plutonic metamorphism now maintained differ 
very considerably from those first enunciated by Hutton. 
The changes which affect the crust superficially, as we have seen, are 
the result of weathering, and are brought about at ordinary temperatures 
and under atmospheric pressure only. But temperature and pressure 
gradually augment with increasing depth. At first they are both 
moderate, and water is plentifully present. Hence the chemical pro- 
cesses taking place in this upper zone might be expected to result in 
the formation of many common minerals, especially hydrates, such as 
hydrous-mica, chlorite, talc, etc., together with magnetite, quartz, calcite, 
etc. To this zone, therefore, should belong such rocks as hydro-mica- 
schist, phyllite, chlorite-schist, talc-schist, serpentine, quartzite, etc. At 
a greater depth the mineralogical changes must become more marked— 
among the metamorphic rocks developed in this second or middle 
zone, would be mica-schist, staurolite-schist and amphibole-schists, 
garnet-rock, mica-gneiss, hornblende-gneiss, marble, quartzite, etc. In 
the deepest zone under a very high temperature and excessive pressure 
the metamorphism ought to be correspondingly increased. Here, owing 
to the meagre presence of water, a general absence of hydrates might 
be expected—and the rocks most characteristic of this zone should be 
gneisses of various kinds (biotite-, augite-, sillimanite-, cordierite-gneiss), 
garnet-rocks, marble, quartzite, etc. In short, the metamorphism would 
gradually increase in intensity as the highly heated interior was 
approached. It is even conceivable that at the greatest depths the 
metamorphosed rocks might be melted. 
Thus the theory of plutonic metamorphism does not, after all, differ 
essentially from that of contact metamorphism, for, according to the 
former, the heated interior of the earth seems to have played the 
same role as a batholith. If the theory in question were generally 
applicable, then it would follow that all rocks which have formerly been 
covered by a great thickness of overlying masses, and thus brought 
within the influence of a high subterranean temperature, ought to be 
more or less metamorphosed; while strata of relatively recent date, 
which never could have been thus deeply buried, ought to be free from 
any trace of metamorphism. As matter of fact, however, there are 
wide regions occupied by great successions of sedimentary rocks—the 
basement beds of which, owing to folding and subsequent denudation, 
