322 Proceedings of Royal Society of Edinburgh. [sess. 
Anorthic Felspars form a very prominent feature ; and of these 
felspars the one which presents the greatest number of interesting 
features is the Aluminium-Sodium Polysilicate, Albite. 
As regards the mode of occurrence of this felspar the fact 
may be stated that it is known to have been formed under at 
least five different sets of conditions. The first of these, and 
probably the most common, is as an original constituent of certain 
eruptive rocks of deep-seated origin. In this connection it is 
found as one of the constituents of certain granites, notably, in 
Scotland, in that of Peterhead, where it is a common associate of a 
potash felspar, of Quartz (usually the smoky variety), and of a 
variety of Biotite containing rather a higher percentage of iron 
than is usual in that species. In the case of such plutonic rocks as 
these, it would probably be safe to say that Albite never shows all 
of its ordinary natural crystalline boundaries, because it has grown 
up from the magma concurrently with the enlargement of its 
associated minerals, through which cause its natural tendency to 
take a definite shape has been hindered more or less. In other 
words, in these plutonic rocks Albite is allotriomorphic, and has 
been obliged, in growing, to adapt its boundaries to those of its 
neighbouring crystals which were doing the same. 
Albite occurs in granites also under somewhat different 
circumstances, and is of much commoner occurrence thus. Its 
history in these cases is not difficult to make out. It is clear that 
during the later phases of consolidation the granite masses gave 
off considerable quantities of vapour, in much the same way as 
lavas do. From this cause many cavities, analogous to the vapour 
cavities of lava, were formed — usually on the outer parts of the 
granite, where it may be presumed that the pressure was liable to 
be occasionally lessened. A druse formed under these conditions 
presented a very suitable nidus for the development of crystals. 
Accordingly, we find in them some of the most interesting minerals 
that occur in native connection with the granites. Amongst these 
may be mentioned Topaz, Beryl, Cairngorm, Pock Crystal, and 
other gem stones. The commoner rock constituents of the granite 
itself also appear in these cavities. A definite order of succession 
prevails amongst these. As a rule, the potash felspar crystallises 
first, then the quartz crystallises upon that, and later than both 
