1893. ] NEW YORK ACADEMY OF SCIENCES. 213 
minerals are so abundant in the rock there can be little doubt 
that they are the source of the serpentine. 
Another phase of the limestone shows, scattered through the 
mass, imperfect prismatic crystals, averaging about an inch in 
length, and dark gray or black. On weathered surfaces these 
erystals project from the rock surface, but are themselves much 
decomposed. Under the microscope the mineral shows the rect- 
angular cleavage, parallel extinction, high interference colors, 
uniaxial figure and negative character of scapolite. The cause 
of the dark color is seen in the presence of great quantities of 
small black inclusions. These are of irregular shape, often 
elongated parallel to the vertical axis of the scapolite. In some 
cases they are quite evenly distributed through the mineral, in 
others, are grouped in patches ; but nearly always a thin outer 
layer of the crystalis quite free from inclusions. Similar inclu- 
sions in scapolite are described by Rosenbusch * as consisting 
of carbonaceous matter and this is undoubtedly true of those 
under consideration. The samerock contains scapolite, usually 
in small grains, which contains no inclusions whatever. Titan- 
ite, pyroxene and micaare also abundant, the two latter some- 
times intergrown. 
Other varieties of the limestone are plenty, but as their interest 
is mineralogical rather than petrographic, their consideration 
is beyond the scope of this paper. 
QUARTZITE, 
The Potsdam sandstone, so far as examined under the micro- 
scope, possesses the character of a very pure quartzite. It is 
made up of well rounded grains of quartz, cemented into a 
tough, compact mass by the deposition of secondary silica. 
Besides quartz no mineral has been found forming complete’ 
grains, though other species are not uncommon as inclusions 
in the quartz. Of the minerals occurring in this way biotite 
zircon, apatite and rutile are the most abundant. Liquid inclu- 
sions are also present in great numbers, but never formed in the 
quartz subsequent to its incorporation into this rock. 
When, as often happens, the rock is red, the color is due to 
finely divided hematite, which forms a thin coating over the 
quartz grains. The hematite clearly was deposited upon the 
quartz before the introduction of the cement, and its appear- 
ance suggests that it is the result of subaerial erosion, formed 
by some such process as that outlined by Russell. + 
* Mikroskopische Physiographio, 3d edition, Mifare etish 
t Russell, I, C., The Subaerial Decay of Rocks, Bulletin 52, U.8.G.8. 
