206 J. P. Cooke, Jr., on some American Chlorites. 
intensity of their color on the two sides of the image, but then 
this difference is constant and after examining a large number 
of plates placed in different positions in the field of the micro- 
scope I was left in no doubt as to the fact. It follows then, that 
while these crystals are monoclinic and the position of the bi- 
sectrix is the same as in the crystals from Texas, the plane of 
the optical axes instead of coinciding with the plane of sym- 
metry is at right angles to this plane. 
Clinochlore from Chester, Massuchusetts—Through the kindness 
of Dr. C. T. Jackson of Boston I have had the opportunity of 
examining some well-characterized chlorite crystals, which occur 
at the emery mine of Chester, Mass., in connection with 
pore and its associated minerals. The crystals are double hexa 
gonal pyramids but very rough and irregular. The have & 
bisectrix inclined to the cleavage plane; and moderate disper 
sion of the axes g>v. Moreover the angle between the optical 
lar to the edge of the hexagonal section, and the dispersion! 
the image is lateral as in the crystals from Chester Co. Pa. 
Indeed these crystals resemble the last in almost every 
except that they are smaller, have a deeper color, and are not $ 
generally macled. 
Analogies between the Chlorites and Micas.— Although these two 
minerals crystallize in very different systems, yet the analogies 
between their crystalline characters is very remarkable. In 
crystals of both minerals the section made by the cleavage plane 
isa regular hexagon; in both, the double refraction is strong 
and the angle between the optical axes is very variable; 10 ar 
we can distinguish two optically distinct varieties, in De 
which the plane of the optical axes has the position shown & 
fig. 3, while in the other it has the position shown in Hg-° 
tly Kiimmererite and Pennine stand in somewhat the same 
relation to the two varieties of clinochlore that Vesuvian Bie 
is to the two co: ding varieties of mica, although " 
must be remembered, that while the double refraction of the wl 
axial chlorite is very feeble that of the uniaxial mica 1s 
a strong. Mineralogists generally admit with DeSenarmont 4 
at the variations in the optical axes of the micas result from 
See: influence of os two optical varieties and the facts of 
