MANHATTAN ISLAND 85
rock, now so familiar to us, yet, in some respects, so difficult of interpretation.
Heat
from presssure, heat from mechanical movement, static heat (the
interior heat of the earth), the friction of the beds over each other,
pressure from the crustal shortening, pressure from superincumbent
masses of sediment, brought about a sort of fusion of the whole, in
which vapor of water at high temperature was disseminated. The mud,
silt, and sands were thus brought under mineralizing agencies which
slowly formed the various silicates now represented by the micas,
feldspars, hornblende, and associated minerals, the unequal and varying
contents at different points making different local products, as the
garnets, where lime prevailed; tourmaline, where iron and boron chanced
to be; sphene somewhere else, where there was titanium, cyanite, beryl,
oxides of iron, col-umbite, and all the rest, according to such
aggregates as, obeying chemical affinities, the deposits warranted.
Not,
indeed, that this explains everything, for one of the mysteries of rock
and mineral making is that very similar chemical conditions produce now
one sort of rocks or mineral, now another, and why, is not easy or
possible to determine.
This action we have described was metamorphism, and it seems probable that the conformable (see ante) granite
veins were formed with the schists and gneisses. But it is conceded the
larger granite stocks were igneous intrusions. In Brittany, in Central
France, in Scandinavia, in Germany, in Canada, granite veins, or
seams, are interstratified with gneiss, or mica-schists, and appear
contemporaneous. Granites vary greatly in composition, the percentage
of silica (quartz) being variable, and thus changing the relations of
the other elements. The coarse and fine granites on Manhattan Island
suggest naturally differing conditions, as perhaps slower
crystallization in the case of the former, though it also seems that
chemical composition has something to do with this contrast of texture.
Now the whole extent of beds was, in the process of this
