PETROGRAPHY— PHONOLITE. 493 



groundmass in crystals having the general form of hexagonal prism and 

 base. In the thin section under the microscope, it is seen in its peculiar 

 hexagons and rectangles, the former being sections parallel to the base and 

 the latter parallel to the vertical axis. Consequently, the hexagons become 

 simply light and dark with parallel and crossed nicols, the polarized light 

 passing through the optic axes, while the rectangles polarize quite brilliantly, 

 being most distinctly seen when the nicols are crossed about 45°. When 

 the crystals are cut obliquely, the section will have an irregular shape, the 

 sides of the hexagon not being equal. This is not caused by any irregu- 

 larity in the crystal, but by the position of the plane of section. Square 

 forms occur, which polarize brightly and therefore are sections cut parallel 

 to the vertical axis from prismatic crystals as thick as they are long. There 

 are generally more hexagons than rectangles, which arises from the fact 

 that the nephelite crystals are arranged with their basal faces parallel to the 

 schistose structure of the rock, while the fragments used for thin sections are 

 most conveniently obtained and ground parallel to the same schistose forma- 

 tion. The nephelite crystals in phonolite frequently have inclusions of 

 microlites, the presence of which in the hexagons as well as in the rect- 

 angles proves the two sections to be from the same mineral. Apatite in 

 section also gives hexagons, but the sections in the other plane are those 

 of long slender needles, while the crystals of nephelite are short and stout. 

 Again, nephelite often make up a large part of the groundmass of phonolite, 

 while apatite occurs in most every case only sparingly in needles. 



Hornblende in small, green columns or microlites is present in almost 

 every phonolite, and these crystals often have forked or split terminations. 

 Besides these, there are also grains, minute in size and larger crystals and 

 masses. 



Nosite has peculiar forms, consisting of a nearly transparent central 

 part, which is often covered with a striation of black lines at right angles 

 to each other like a net- work, and a dark, opaque border, quite irregular in 

 shape. It is isotrope, changing simply from light to dark, but its peculiar 

 and striking appearance causes it to be easily recognized. It is also dusty 

 or filled with a great number of dark dots, as if shaded by points. This is 



