EL. 8. Dana—Chondrodite from the Tilly-Foster Iron Mine. 108 
given in the various tables, show that the variation from the 
rectangular type, if it really exist, must be very slight, as the 
agreement between the angles measured and those calculated 
on the assumed prismatic basis is very close—it being remarked 
that some considerable variation in the angles given in the 
tables are simply explained by the impertection of the crystals. 
Note the angles measured for m? Am? on the twin crystals de- 
scribed on page 96. It was not to be expected that the varia- 
tion i2 the optical character of the crystals would be so decided 
in view of the slight divergence which is possible in the erys- 
talline form. I reserve for the future the careful revision of 
The axes, as already mentioned, do not appear distinctly ex- 
cept in oil; in the first mentioned section they admitted of 
good measurements. The mean of thirty determinations of 
the angle for red rays gave— 
2 Ha=88° 48’: the extremes being 88° 36’ and 89° 0’. 
With a yellow light (sodium) the angle was essentially the 
same, but the mean was 10’ or 15’ smaller, which would indi- 
cate that the dispersion is p>v, but the matter cannot be con- 
sidered to be beyond doubt. 
The index of refraction of the oil employed, as determined 
by Professor Wright and myself, was 1°466. 
In conclusion, I have to express my very great obligations 
to Prof. Allen for his kindness in giving me free use of all the 
Wr in his valuable cabinet. Both of the crystals of the 
third type, as well as several others mentioned, came from hi 
collection; in fact it was Prof. Allen who first made known 
the special interest connected with the locality. To Mr. Cos- 
griff, the superintendent of the Tilly-Foster Iron Mine, I am 
also much indebted for his uniform kindness and courtesy to 
me at the several occasions when I have visited the mine, and 
also for the gift of several fine specimens. 
