102. EF. S. Dana—Chondrodite from the Tilly-Foster Iron Mine. 
Guided by the observations on the optical properties of hu- 
mite made by Descloizaux and given in his Mineralogy (p. 14), 
to CO (i-7, 010); 2d, that this bisectrix is positive; 3d, that the 
optic-axial angle is large, the axes being seen only when oil is 
used ; but 4th, that the axes do not lie in the basal plane, but in 
a plane making an angle of about 154° with it. This last point 
Was so unexpected and anomalous that every effort was made 
to explain the measurements in some other way, but with no 
success. By means of a stauroscope, made by Fuess in Berlin 
after the excellent pattern of Groth, the position of the two 
axes of polarization, as referred to e', and also to e? in plane C, 
were carefully determined. The measurements were repeated 
twenty times, the error arising from an imperfect adjustment of 
the Nicols being eliminated in the usual manner. ‘The result 
was as follows: 
Supplement angle made by the plane of the axes— 
with e' (3-4—203), 18° 9’; hence with the basal plane, C, 25° 50’. 
with e* (24=201), 45° 9’; « «“ «“ C, 25° 46’ 
T 
? 
In order to confirm these results, other crystals were sought, 
which would admit of like determinations. None could be 
found which would serve for measuring the axial angle; but 
two small ones, on which the plane C was naturally developed, 
proved to be clear enough to allow of measurements with the 
stauroscope. The first alone gave accurate results; on it the 
angle of the same plane with e*(2-7= 205) was determined with 
equal care. The results were: 
4° 55’ for the angle with e*; and hence 25°59’ with ©. 
The agreement with the angles given above is as close as 
could be desired. In the other case, the rather rare plane 
B(i-i=100) was present; the crystal was minute, however, and 
the determination only approximate. It was found that the 
normal to the axial plane made with Ban angle of 65°-70°, 
questioned, and it remains to reconcile it with the crystallo- 
BaPhis properties of the species. It will be seen at once that 
the position of the Optic axes is totally at variance with the 
