in
the Borough of the Bronx), of Cambrian age; the Stock-bridge limestone,
of Cambro-Ordovician age, made referable to the Kingsbridge limestones
on New York island; the Berkshire schist of Upper Ordovician age
(Hudson River), and represented in the prevalent Manhattan schists and
gneisses.
The
mineral composition of these rocks embraces quartz, mica, feldspar,
hornblende, dolomite, with a numerous assemblage of accessory
minerals, and displays contrasted or varied aspects of texture,
solidity, or position. They are related to the extended development of
the crystalline rocks in New England, which, if regarded as original
sediments, shore deposits, or unconsolidated mineral accumulations,
have put on a lithological phase of construction in which their first
or earlier state and stratification have completely disappeared or been
radically modified. This change has supervened through the agency of
metamorphism. As Prof. H. E. Gregory has said: "An explanation of
sedimentary rocks requires a knowledge of the forces operating at the
present time on the surface of the earth; it is necessary to understand the action of rivers, wind, ice, etc.; a complete understanding of the crystallines involves a knowledge of the forces which are at work zvithin the interior of the earth, as well as an understanding of the chemical and mineralogical composition of the rocks as they exist."
The
development of such crystalline masses means a long history. If the
original sediments were muds or granular mineral aggregates, or if the
original rocks were lavas, they underwent initial changes into
crystalline complexes, which again under strain, pressure, and heat,
assumed new mineral constitutions. The mineral feldspar can become
changed into quartz and muscovite mica, or, with added magnesium and
iron elements, into quartz and biotite mica, or into quartz and
chlorite, the free quartz in such cases being supplied by the large
percentage of silica in the feldspar (65 per cent.), exceeding by
almost 20 per cent, the amount of silica necessary for the chemical
composition of muscovite.. Hornblende
