80 
AMERICAN GEOLOGY. 
the limestones under consideration are metamorphic—the lower 
Silurian altered by heat. That this view is incorrect appears 
from the fact, that in St. Lawrence county the Potsdam sand¬ 
stone actually overlies it. Fig. 9 represents a bluff of lime¬ 
stone and other primary rocks at Port Henry, in Essex county, 
New York, against which the Potsdam and calciferous sandrock 
reposes. The relation of these masses is far from being in 
accordance with the metamorphic doctrine. 
Insulated masses of horn¬ 
blende often occur in primary 
limestone. Sometimes they 
appear in quadrangular shapes, 
and in other cases their forms 
are irregular, as represented 
by fig. 10. 
Fig. 10. 
a Limestone, b Hornblende. 
The same view of the origin of limestone is supported by 
the occurrence of it in hypersthene rock at Long pond, in 
Essex county. It is an oblong mass sixty feet wide, extending 
nearly north and south down the face of a precipitous ledge of 
rock. It is rich in pyroxene and scapolite. 
The metals, their oxides or sulphurets, though not very com¬ 
mon in this limestone, have usually run out, and like the granite 
associated with it, it has proved an unsafe rock for mining. 
There is one exception, however, in the franklinite and red 
oxide of zinc. The specular oxide of iron, which is common 
and very beautiful in St. Lawrence county, is often insulated 
and removed entirely from its bed; and it has been as yet 
impossible to recover the bed or vein when once lost. 
The condition of the simple minerals in limestone is worthy 
of special notice. The phosphate of lime, though softened with 
difficulty in the flame of the common blowpipe, is apparently 
acted upon by heat; the edges and angles are rounded or 
flattened, as if it had been in a pasty state since it had assumed 
its prismatic form. The quartz, which is often imperfectly 
crystallized in its usual form, has its angles and edges rounded 
