MINERALOGY. 55 
planes parallel to this plane, are immense numbers of little brown scales 
or rhombic plates. The true nature of these scales has not been deter- 
mined, but they have been suspected to be brookite by some—a conclu- 
sion doubted by others. It is only known that they impart the metallic 
copper color to hypersthene, and that they are inlaid in the determined 
plane in three directions, one of which is nearly parallel to the vertical 
axis, one at right angles to it, and one making an angle of about 30° 
with it. 
Turning now to our hypersthene, it may be noticed that in thin sec- 
tions it is much lighter in color than that occurring elsewhere, and, indeed, 
it is nearly colorless in thin sections. It occurs in irregular fragments 
and grains in the rock, giving no sign of external crystal faces; but when 
in a microscopic examination of a thin section, the vertical axis, as indi- 
cated by the cleavage, is brought parallel to the plane of vibration of 
either one of the crossed Nicol prisms, the section is dark, and does not 
disturb the interference figure of a calcite plate put on the ocular under 
the upper Nicol prism. Hence, it is orthorhombic. The interpositions 
are like those in the St. Paul’s Island hypersthene. It contains magne- 
tite in irregular fragments irregularly distributed, and brown scales much 
darker in color, however, than those in most hypersthene, and which are 
symmetrically arranged. The larger part are nearly parallel to the ver- 
tical axis, but not exactly, for, as was observed by Kosman, the plane of 
the interpositions builds an angle of 7° 45’ with the cleavage plane. 
The second part are inlaid, with their long edges making an angle of 
about 30° with the cleavage lines, and a few scales are apparently inlaid 
at right angles to the first. A peculiarity of this hypersthene is the 
large number of interpositions making the oblique angle to the vertical 
axis. Fig. 2,on Pl. 4, represents the appearance of the hypersthene in 
the Waterville rock, and the mode of arrangement of its characteristic 
interposition. It is not abundant, but it is very conspicuous in some 
sections of the rock, and is very easily distinguished from the pyroxene, 
olivine, etc., with which it is associated. 
40. Prroxene [R Si O,]. 
R standing for Ca, Mg, Fe, or Mn. 
Pyroxene is found at some localities in fine large crystals, and it is 
