326 Proceedings of Royal Society of Edinburgh. [sess. 
in the developmental history of the crystals, has not received the 
amount of attention it seems to deserve. I am not aware, at 
present, that it has been noted at all. Yet it is quite a common 
feature of crystal aggregates of many different species. 
Perhaps it has been passed over owing to a certain reluctance on 
the part of both mineralogists and biologists to admit the existence 
of too close a parallel between organic processes and the forces 
which regulate the growth of a crystal. 
It may be remarked now that there is not a single feature to 
which attention has been called in the case of Albite that does not 
find its parallel in the case of many other minerals. Polysynthetic 
twinning, for example, is by no means confined to Anorthic 
minerals. Much of what passes for simple parallel growth in some 
Orthorhombic species may really be of this nature ; and it may be 
found in crystals belonging to other systems as well. 
In Albite the striations, which are the outward manifestations 
of polysynthetic twinning, are due to alternate salient and retiring 
angles formed by the anorthic angles of the adjacent sub-individuals. 
These, in the aggregate, often give rise to definite angles which 
quite commonly agree with the true crystallographic angle proper 
to two adjacent faces. It is a curious fact to be noted in this 
connection, that many species of minerals show these aggregate 
forms. In Gypsum, for example, the unit prism {110} is fre- 
quently represented by an aggregate formed by the edges of several 
other forms in the prism zone, amongst which, occasionally, the 
unit prism itself may be conspicuous by its absence, and yet the 
angles -agree with those of the form which is simulated. Many 
other instances of the same kind could be cited. In all of these, 
as in the case of the coplanar faces of the aggregates noted above, 
there seems to have been what might be regarded as a kind of 
agreement amongst the components of the aggregate to grow out 
together to one'definite plane, and there to stop. 
It may be remarked here that the different grades in the 
developmental history of such a crystal as Albite have not yet all 
received suitable designations. It may be safely remarked that 
such terms will be needed as soon as mineralogists come to 
recognise the important bearing these facts must have upon many 
problems which await solution relating to crystal growth. 
