324 Proceedings of Royal Society of Edinburgh. [sess. 
the cases where the Albite has grown upon the potash felspar, it 
not uncommonly selects some definite form, such as the unit 
prism {110}, and grows exclusively upon that. There would thus 
appear to he some greater surface tension between certain faces of 
potash felspar and the solutions tending to deposit Albite than 
there is in the case of other faces. Gravitation has nothing to do 
with this selective action, because the faces in question may lie in 
any direction with respect to the horizon. The feature in question is 
well shown upon specimen 316/33 of the Scottish Mineral Collec- 
tion,* which is from Sterling Hill, Aberdeenshire. I shall endeavour 
to give fuller details further on in support of the view here advanced, 
that variations in the magnitude of the surface tension between 
the solutions and certain faces upon which deposition is in progress, 
are responsible for the remarkable selective action which has 
evidently regulated the growth of many crystals. 
The next point for consideration in connection with the Albite 
crystals relates to their growth. It is probable that few, or no, 
Albites are entirely free from traces of Calcium ; though, in general, 
the proportion is below two per cent. The usual theory regarding 
this calcium is that it exists in combination within the Albite as 
units (or, as some would regard it, as molecules) of the Aluminium- 
Calcium Polysilicate, Anorthite. It is usually considered that 
there is a progressive change in composition from Albite through 
Oligoclase, Labradorite, etc., to the pure lime felspar Anorthite ; 
the minerals just mentioned being only two c species ’ arbitrarily 
chosen out of an almost perfect series of intermediate gradations 
between the two extremes. It will be convenient to assume, for 
the object at present in view, that the Scottish Albites under 
notice are of this compound character, and are not absolutely pure. 
We may here recapitulate the points under consideration : — 
The stages so far noticed in the developmental history of Albite are 
represented by (1) the chemical atoms composing each of the Units 
of Substance ; (2) the separate units of Soda Felspar and Lime 
Felspar ; (3) the substance formed by the admixture of these two. 
Beyond this point many who have speculated upon the subject 
* The registration marks, or other means of identifying the specimens 
referred to, will be given in every case cited in these papers, so that the 
statements may be independently verified when anyone wishes to do so. 
Mineralogists should agree to give similar references in all cases. 
