MELLO: MICHO^COPICAL STRUCTURE 01" ROCKS. 
167 
prisms exhibit a remarkable series of parallel stripiugs of a dark or 
bluish grey tint, intersecting each other at an angle of 60 ' or \H)^. 
On rotating the analyser the dark bands will become clear, and the 
bluish-grey will present the usual phases of doubly refracting crystals. 
Leucite generally contains very numerous enclosed microlites of 
felspar and augite, and gTanules of magnetite, etc., which are 
frequently found arranged in concentric rings or symmetrical zones, 
lying sometimes parallel to the boundaries of the crystal. 
Nepheline, a frequent mineral in volcanic rocks, plays a 
very important part in some basalts and lavas. It occurs very 
generally in hexagonal or rectangular forms. The hexagons l)eing 
cut at right angles to the vertical axis will not polarisi'. but 
sections which are parallel to it polarise with a brownish-yellow or 
light greyish-blue colour. What appears to be a tine grey dust is 
sometimes seen in nepheline crystals, which, with a high power, is 
resolvable into glass or fluid cavities. Very small green microlites, 
parallel to the long axis, are also found in nepheline. The enclos\ires 
are sometimes densely accunuilated in the centre of the crystals. At 
other times they are seen to occupy zones parallel to their boundaries. 
Occasionally nepheline crystals exhibit aggTegate polarisation, and 
entire crystals will be found converted into an aggregate of zeolites. 
An oily-looking green or pinkish variety of nepheline, which is 
seldom distinctly crystallised, is occasionally found to take the place 
of this mineral in the older igneous rocks, and may be considered in 
reference to nepheline, to be what othoclase is to sanidine, it is 
known under the name of Eheolite. 
We may, in the next place, notice Apatite. It is .some- 
what difficult to disting-uish between apatite or nepheline, but 
where the two minerals occur together it will be observed that the 
hexagonal crystals of nepheline are somewhat larger than those of 
apatite; and it may also be noticed that wliilst nepheline occurs in 
short colourless rectangles, the apatite will form long colourless 
needles; both minerals are hexagonal, and by this may be distin- 
guished from minute sections of felspar. 
Sodalite is a representative of a group of isomorphic regular 
minerals found in volcanic rocks. Microscopic sections show 
