diorite
(igneous) intrusive rock. This alteration forms "a purely biotitic
gneiss or schist, or spangled or mottled micaceous gneiss, carrying
both black and white micas. The latter differs from the prevailing
gneiss of the Manhattan series, generally in a somewhat finer texture,
greater richness in micas, and retention of many scales of both micas,
which are long and bladed like those of the antecedent hornblende.
Excellent examples were found at East 64th Street, on the East River;
West 85th Street, just east of 10th Avenue; East 99th and 1 ooth
Streets, between Lexington and 4th Avenues; West 108th Street and
Riverside Avenue; West 127th Street, near St. Nicholas Avenue; West
165th Street, on path above and Speedway; West 190th Street and
Amsterdam Avenue.
"Through
its great plasticity this biotitic or micaceous gneiss often bends
about and incloses the bunches of less altered hornblende gneiss in a
manner somewhat resembling a flow structure. Prominent localities were
noted at West 90th Street, between nth and 12th Avenues; West 92d
Street, near the Hudson River; West 141st Street and 7th Avenue. At
West 58th Street, between 9th and 10th Avenues, a group occurred of
four thin layers of slaty hornblende gneiss and one of black biotite
gneiss, separated by layers of micaceous gneiss. At 57th Street, only
200 feet farther south along the strike, this entire group was
represented by a single thick bed of black biotitic gneiss."
By
anyone accustomed to refer the hornblende intercalations to the
metamorphosis of ferro-magnesian sediments it would seem entirely
reasonable that contiguous (in this case over and underlying) beds
would also contain iron and magnesia, mineralized, when the iron was
low and the magnesia high, into biotite.
