BERKEY, GEOLOGY OF SOUTHERN MANHATTAN ISLAND 249 
member. Each of these exhibits a considerable variety of composition, 
texture and structural quality, and two especially — the gneisses and the 
schist — are, in some cases, so nearly alike in appearance that they are dis¬ 
tinguished with great difficulty. This is particularly true in cases where 
small outcrops must be relied upon, or where there are no outcrops, and the 
information is confined to fragments recovered from borings. The difficulty 
is still greater where the chop drill has been used and the rock broken to 
very small bits, or grains. In some of these cases, it is the writer’s opinion 
that it is impossible to identify every specimen. A thorough acquaintance, 
however, with the possible range of petrographic characters will enable one 
to identify any reasonably typical specimen, and where several from the 
same general locality are available, there is never any serious doubt of the 
identification. In the older records of engineers and some of the city de¬ 
partments, very general terms are used for bed-rock, the terms granite, 
schist and gneiss being used rather promiscuously for the crystalline base¬ 
ment without distinguishing between the different large formations. In a 
study of this material for the unravelling of the structural geology, however, 
it is essential to identify the specific formation to which each specimen 
belongs. The following descriptions may serve as a guide for such determi¬ 
nation. 
Fordham Gneiss 
The most characteristic Fordham Gneiss is a close-textured, black and 
white banded quartz-feldspar-hornblende-mica rock of a general composi¬ 
tion similar to a very quartzose granite. The bands are seldom more than 
an inch or two in width, rather persistent, frequently crumpled or folded 
and usually hard and durable. There is a general foliated structure which, 
in some places, is not very noticeable and in others is very strong. This 
typical Fordham is easily identified, but large areas and numerous belts or 
streaks of this formation exhibit no banding at all. On the contrary, they 
are comparatively massive and have all of the characters of a gneissoid 
granite, diorite or granodiorite. Less commonly the rock exhibits a strongly 
quartzose character, becoming a quartzite schist. Associated with this 
type may be found a more strictly mica or hornblende schist of uniform 
structure, and still more rarely very impure limestones interbedded with 
the gneisses are found. The banded type is very prominently exposed in 
the northern part of Manhattan Island along Seventh Avenue and is still 
more strongly developed on the east side of the Harlem River in the Bronx. 
The massive types are much more common in the exposures in the East 
River, Long Island City, Brooklyn and in certain drill borings of southern 
