18 GEOLOGY OF NEW YORK CITY
An
excellent example of progressive metamorphism from the Lake District in
the north of England is thus delineated by Geikie: " The slate where
unaltered is a bluish-gray cleaved rock, weathering into small flakes
and pencil-like fragments. Traced toward the granite, it first shows
faint spots, which increase in number and size until they assume the
form of chiastolite crystals, with which the slate is now abundantly
crowded. The zone of this andalusite-schist seldom exceeds a quarter of
a mile in breadth. Still closer to the granite a second stage of
metamorphism is marked by the development of a general schistose
character, the rock becoming more massive and less cleaved, the
cleavage planes being replaced by an incipient foliation due to the
development of abundant dark little rectangular or oblong spots,
probably imperfectly crystallized chiastolite, this mineral, as well
as andalusite, occurring also in large crystals, together with minute
flakes of mica (spotted schist, knotenschiefer). A third and
final stage is reached when, by the increase of the mica and
quartz-grains, the rock passes into mica-schist—a light or bluish-gray
rock, with wonderfully contorted foliation, which is developed close to
the granite, there being always a sharp line of demarcation between the
mica-schist and the granite."
In
the French Pyrenees and in the Eastern Vosges similar progressive
changes in the clay-slates are traced towards the granite, being at
first colored and fissile with quartz stringers, then growing spotted,
lighter in color, and harder, with swarming mica plates, then
graduating into a ringing stone, highly crystalline and made up of
andalusite quartz and mica. (Andalusite, as a silicate of aluminum,
naturally is an end product in these changes from an original clay bed.)
As
accentuating the metamorphic character of the Manhattan Island beds, it
has been insisted that as we pass northward the crystalline schists are
gradually replaced by slates. These slates are Hudson River beds, and
their correlation with the Manhattan schists makes the latter of the
same age. This is called " Progressive Metamorphism," and has been
illustrated
