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monoclinic system, and on clearing the crystals, cracks are often 
visible, traversing the clinopinakoid faces and denoting the easy 
cleavage in the direction «Pao. The cleavage of the basal 
planes OP is very perfect, or as it is important for our present 
purpose, the cleavages are after Bauerman, “001 very perfect, 
010 perfect, 110 imperfect,” and are often better parallel to one 
pair of faces than to the other. The optic axial-plane is usually 
perpendicular to 010, the first median line inclined at 111° or 
112° to c, or 5°—6° to a, giving the horizontal dispersion. By 
strongly heating them the angle of the axes in 010 is increased, 
while that for the plane perpendicular to it is diminished. If 
the heat exceeds 500°, the original positions are not quite 
recovered on cooling. 
The granite contains besides Orthoclase, another in rather 
less quantity; this is Albite, so called from its white colour, a 
soda felspar crystallising in the triclinic system, the soda is 
very commonly replaced, to a small extent, but not above 2°5 
per cent. by potash, and twinned crystals are the rule, not the 
exception. The thin lamelle of the tabular forms are often 
repeated in parallel directions, and the intimate structure of the 
twinning planes shows at the edges an irregular surface ; more- 
_over Albite occurs not seldom in a distinctly polysynthetic, 
granular state, and some varieties reveal, but not so markedly 
as in Orthoclase, a network of meshes, which, at least in the 
case of the latter, cross each other at right angles. 
Albite fuses more readily than Orthoclase;* but when 
melted together, they fuse much quicker than apart from each 
other; thus these alkaline felspars comport themselves like the 
alkaline carbonates, a fact which chemists turn to practical 
account in assaying, by depriving them of their water of 
crystallisation, pulverising and keeping the mixture as a flux of 
general application; since a mixture-of both is far preferable 
to either alone, and besides, requires a lower heat for its fusion. 
When therefore a granite contains a potash felspar side by side 
with a soda felspar, and is exposed to a high temperature, the 
consequence is obvious, as we shall see in the case of the 
* Orthoclase being K, Al, Si, O,,, and Albite Na, Al, Si, O,,. 
