Garnet at the Spurr Mountain Iron Mine. 19 
parallel to the undulation plane of the nicol, to very light 
(with greenish-yellow tint) when perpendicular to that plane. 
ssuming that the parallel sides of the bands are crys- 
tallographic outlines and that they lie either in the basal 
plane, or else parallel to the principal crystallographic axis, 
I have attempted to determine optically the system to which 
these crystals belong. The method followed is that recom- 
mended by Tschermak in distinguishing pyroxene, hypers- 
thene and biotite. Having carefully adjusted the microscope,* 
so that the cross hairs in the ocular coincided exactly with the 
stage till the nearest point of maximum darkness was reached, 
when the principal sections of the crystal coincided each with 
a principal section of a nicol’s prism. e number of degrees 
of this revolution indicate the inclination of the principal sec- 
tions of the crystal to the crystallographic feature chosen for 
reference. : 
Of course, if the mineral were either uniaxial or orthorhom- 
bic, the maximum of darkness would occur when two of the 
axes of the crystal were parallel to the undulation planes of 
the nicols, and there would be no revolution required. But 
this could occur in a monoclinic crystal only when one of the 
principal sections happened to coincide with the plane of sym- 
metry ; in every other position the axes of the erystal would 
make with its principal sections an angle which would vary 
between 0° and the number of degrees representing the 
inclination of the bisectrices to the vertical and inclined lateral 
axes; the full amount of this inclination could only be ob- 
served when the principal sections of the crystal were perpen- 
dicular to the plane of symmetry, but any inclination suffices 
to determine that the crystal belongs to a clinobasic system. 
Observations on a great number of the bands and isolated erys- 
tals failed to show any inclination ; the mineral, therefore, does 
not belong to a clinobasie system. 
IL The clear green portions occur isolated in the garnet 
fragments, and in places in the fissures with the mineral last 
described, and more or less diffused through the larger bands, 
but more Roweagl in irregularly-shaped spots, and with out- 
lines which are not necessarily determined by those of the 
garnet fragments. These larger areas exhibit lamellar aggre- 
gate polarization both between crossed nicols and with the 
Polarizer alone. Between crossed nicols portions remain 
*One of Beck's first . : : . : ; 
etl Mhuthbaa 
