354 T^Jic American Geologist. December, lyoo 
without the microscope. On decomposition the rock first be- 
comes rusty, as hematite is formed on the surface; then it be- 
comes dark greenish as the fayahte ahers to bowHngite, etc. 
Texture. Under the microscope the siUcoferrolyte is 
found to have a marked poikiHtic texture, large crystals of 
fayalite enclosing many magnetite grains, which very rarely 
show any crystal outline. Between the fayalite masses occur 
anhedra of pyroxene, only rarely surrounding the former be- 
cause only rarely abundant enough. They may enclose, or 
be enclosed by, the fayalite. But the feature of the rock is 
the presence of many rounded grains of quartz scattered every- 
where in the rock. These quartz grains are usually very 
small; they often present concavities, perhaps of the nature of 
embayments. They occur in multitude enclosed within the 
other minerals. (See plate XI, figure i.) The orientation of 
these grains is wholly heterogeneous and this reenforces the 
poikilitic texture. The order of crystallization is: 
Quartz. 
Magnetite. Hercynite. 
Fayalite. 
Pyroxene. 
No feldspar is found in the rock. The secondary minerals 
are neither numerous nor abundant, they include: actinolite, 
tremolite, bowlingite. gothite, hematite and the chlorites. 
Constituent Minerals. Quartz is such an un- 
usual element of an ultrabasic rock that the diagnostics de- 
pended upon for its determination will be specified with care. 
It presents no crystal outlines, being always round or irrei^u- 
lar. It shows no cleavage and no twinning. Undulatory ex- 
tinction is very rare. The mineral is of a glassy transparency 
and perfectly colprless. The refringence is low, and n^'x's, in- 
ferior to the refringence of Canada balsam, but n^ is superior 
to the latter. The birefringence is .009. Sections directly 
perpendicular to ;/g show that the mineral is strictly uniaxial 
and positive. 
Liquid inclusions occasionally containing moving gaseous 
bubbles are not rare; they occur either irregularly, or along 
curving planes. 
Magnetite is the most abundant component of the rock. 
It very rarely shows crystal outline; the octahedral parting, 
