ON THE MICROSCOPICAL STRUCTURE OF ROCKS. 
15 
already been noticed, under the name of lamellar polarisation. 
Between crossed prisms the leucite crystal will exhibit a, re- 
markable series of parallel stripings, of a dark or bluish grey or 
blue colour, crossing each other at right angles, or at an angle 
of 60°. On the rotation of the analyser the black or dark 
stripes will become clear, and the blue grey presenting the usual 
phases of double-refracting crystals. Zirkel has considered this 
curious optical effect to be owing to an intergrowth of two 
different species of leucite, one of which is double refracting. 
Another writer, Yon Bath, shows that the leucites have a great 
tendency to twin formation. Analcime is another mineral which 
exhibits a somewhat similar lamellar polarisation, and which 
has been referred to as a laminated structure, in which the 
la mi nae are separated by thin strata of air ; and by some ob- 
servers the behaviour of leucite has been set down to this kind 
of structure. Another distinguishing feature of leucite consists 
in the abundance of enclosed microlites it contains, sometimes 
irregularly arranged, but more frequently found in central 
groups or in concentric rings of stone cavities, near the circum- 
ference of the crystal. Some of these microlites will be found 
to be augite needles, and grains of a greenish or yellowish 
colour, others grains of magnetite. Besides the stone cavities, 
leucite also contains numerous gas and glass cavities, the latter 
often marked by diminutive immovable bubbles. 
Nepheline. — 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 being cut at right angles to the axis will not polarise, 
but sections which are parallel to it polarise well with a light 
brownish yellow or a light greyish blue colour. What appears 
to be a fine grey dust is sometimes seen in nepheline crystals, 
which with a high power may perhaps be resolvable into glass 
cavities. Very small green microlites, parallel to the long axis 
of the crystal, are also found in nepheline. Although nepheline 
is a very stable mineral, yet sometimes its crystals will show an 
aggregate polarisation, and the whole crystal will occasionally 
be found converted into an aggregation of zeolites. An oily- 
looking, flesh or green coloured variety of nepheline, which is 
seldom distinctly crystallised, is occasionally found to take the 
place of nepheline in the older igneous rocks ; this is elseolite, 
and may be considered in reference to nepheline to be what 
orthoclase is to sanidine* 
Apatite . — It is somewhat difficult to distinguish between 
nepheline and apatite, but careful observation will show that 
where these minerals come together so as to be compared, the 
hexagonal crystals of nepheline are somewhat larger than those 
of apatite ; and it may also be observed, that whilst nepheline 
