MINERALOGY. 55 



planes parallel to this plane, are immense numbers of little brown scales 

 or rhombic plates. The true nature of tlicse 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 hghter 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 PI. 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. Pyroxene [R Si 0^]. 



R standing for Ca, Mg, Fe, or Mn. 



Pyroxene is found at some localities in fine large crystals, and it is 



