76 
AMERICAN GEOLOGY* 
the veins of segregation, and the rock also cracks into wedge-* 
form masses. 
Like other granites, this rock decomposes, and forms a clay, 
which is quite refractory in the fire; hut it is never so white as 
the purest porcelain clays. In this natural or spontaneous 
analysis, most if not all of the granites furnish, in connection 
with the pure alumina, oxide of iron, peroxide of manganese, 
and crystallized silica. 
The hypersthene rock is confined mostly to northern New 
York. The western part of Essex county is made up entirely 
of this rock. It forms all of that group of mountains in this 
part of the state which are known as the Adirondacks. A 
train of bowlders, derived from this cluster of mountains, 
passes through Amsterdam, thirty miles west of Albany. It 
extends to Orange county. Another train of bowlders range 
along the St. Lawrence in St. Lawrence county, New York, 
This train came from another group of mountains far to the 
north or northeast, probably Labrador. It has no connection 
with the Adirondacks. 
This rock receives a fine polish, and would form beautiful 
tables and other ornamental articles of furniture. The most 
important mineral associated with this rock is the magnetic 
iron ore. Prehnite, chalcedony, and albite are found in this 
rock, though by no means abundantly. It is poor in minerals. 
It contains subordinate beds of pyroc^stalline limestone, which 
are rich in minerals: those, for example, which are common to 
it when it is associated with other rocks. 
The area which this rock covers is small when compared 
with the common varieties of granite. In the United States it 
is mostly confined to the region occupied by the Adirondacks: 
though I have observed a few small patches in other parts of 
New York, they are too inconsiderable to require a notice in 
this place. It is impossible to determine even approximately 
the age of this rock. It is isolated and disconnected with 
fossiliferous rocks. The elevation of the Adirondacks, how- 
ever, w r as probably subsequent to the consolidation of all the 
