1901-2.] J. G. Goodchild on Scottish Mineralogy. 
327 
Reverting to the modes of occurrence of Albite, we find that 
this mineral occurs, sometimes in large masses, as one of the 
constituents of the pegmatites which form part of the Archaean 
gneisses of Scotland, especially in the Hebrides and in Shetland. 
These gneisses appear to have originally been various eruptive 
rocks of deep-seated origin, such as granites, diorites, gabbros, and 
others. They have all undergone more or less deformation, as a 
result of differential movements throughout the mass, caused by 
crust-creep, which is probably one of the manifestations of the 
same terrestrial forces to which the elevation of continental areas 
and the depression of ocean basins have nearly always been due. 
The rock materials have been crushed and drawn out into a 
laminar arrangement by the earlier movements, which took place 
under great superincumbent pressure, and in the presence of water. 
When this movement has been repeated (if the rocks have not 
been annealed by exposure to a high temperature in the mean- 
time) the constituents have become granulitic, just as metals would 
under like circumstances. At a later period reconstruction has 
taken place, in the presence of concentrated sea water, I think, 
and the constituents, perhaps as a consequence of a temporary and 
local relief of part of the pressure, have recrystallised. The 
process takes place through the formation of ‘ augen,’ in which 
each ‘ eye ’ consists of a separate mineral, which has enlarged 
apparently by the migration in the direction of each ■* eye 5 of 
constituents derived from the rock adjacent. If, as seems probable, 
the process took place under conditions which permitted of the 
introduction of additional alkalies derived from the sea water 
carried in by osmosis, the percentage of alkali silicates, Albite 
amongst others, may well have been increased. But these are, at 
present, little more than hypotheses, which, though they appear to 
harmonise with the facts, may not in all cases be the true explana- 
tion. Be that as it may, the largest masses of Albite occurring in 
Scotland are found under the conditions above mentioned. They 
are never idiomorphic, and there is, therefore, no means of deter- 
mining what their developmental history as crystalline masses has 
been. They all, however, show traces of polysynthetic twinning. 
Under subsequent strain arising from movements after the final 
stage of consolidation, a further change has affected the Albite 
