158 
MELLO: MIOROSGOPICAL STRUCTURE OF ROCKS. 
Amongst minerals which may become dark between crossed 
Nicholls, when cut at right angles to an optic axis, is the mica so 
abundant in sections of granite, but rotation will at once show that 
the dark object before us is no glass, glass remaining under all changes 
of position colourless. 
Such structural peculiarities as twin crystallization are clearly 
and beautifully defined by the use of the polariscope, the twinned 
crystals being distinguished by different and complementary colours, 
thus we have a ready mode of determining in many cases whether a 
felspar in the rock under examination belongs to the orthoclastic or 
plagioclastic series, the latter being very usually, though not invari- 
ably, divided into more or less numerous parallel twin crystals, which, 
as for instance in labradorite, give the crystal a very beautiful rib- 
bon-like appearance, the stripes being usually exquisitely contrasted 
complementary tints. Orthoclase felspars, though often twinned on a 
larger scale, are not minutely twinned as are the triclinic varieties. 
There is one remarkable form of twin structure which is called 
lamellar. Crystals of leucite and of boracite exhibit lamellar polari- 
zation in a very perfect manner, the sections of these minerals are 
seen to be very definately striated, the stri?e instead of being all 
parallel, as in the felspars, cross each other at a certain angle, having 
a kind of basket-w^ork appearance. 
I have already alluded to the presence of minute crystals em- 
bedded in the substance of the larger ones, careful observation of 
of these, as of the base in which the larger crystals are found, will 
parallel and at right angles to the crystallographic axes (sections lying in 
the zone of the orthodlagonal) there are others in which this is not the 
case, the mineral is monoclinic. 
5 When all the sections behave similarly and may be pleochroic, it is rhombic. 
6 If the principal directions of vibration are neither parallel nor at right angles 
to the crystallographic axes and pleochroism occurs, we have a monoclinic 
or a triclinic mineral. 
7 If all the sections do not behave alike and in some the principal directions of 
vibration are not parallel or at right angles to the crystallographic axes, 
the mineral is monoclinic. 
8 If all the sections behave alike, it is tricHnic. 
