1901 - 2 .] J. G. Goodchild on Scottish Mineralogy. 
325 
would be inclined to place the Units of Structure, by whose 
arrangement the sub-individuals which now come to be considered 
have been built. Be this the correct view or not is immaterial. 
But (4), the stage of the sub- individuals, is evident enough. The 
whole of the plagioclase felspars, from Albite to Anorthite, have 
long been known to be made up of laminae of varying thickness, 
but which are usually very thin, and rarely exceed a millimetre 
in thickness. Each of these laminae generally shows the commonest 
unit forms of the complete crystal, i.e. {010, 110, llO, 001, 101 
and T01 }, but rarely other forms than these. Each of these sub- 
individuals has grown next to another sub-individual, which has 
been, as it were, rotated, through 180° around an axis coincident 
with {010}. (5) These Polysynthetic Twins are firmly united into 
what one may regard as a crystal unit, or, as it will be called here, 
Individual, which, in addition to the crystal forms just mentioned, 
may show others which are never developed upon its components. 
(6) The developmental history does not stop at this. In many 
cases one individual is again united with another, either individual 
being invariably rotated, in relation to the other, through 180°, 
around one or other of several different axes, thereby giving rise 
to Twins, which are usually doublets, but may be triplets or 
higher combinations than those. It is remarkable that the plane 
with reference to which the two individuals become symmetrical 
after the revolution referred to in no case coincides in direction 
with a plane of symmetry for the individuals themselves. This, 
of course, is not manifest in the Anorthic System, though it is 
clearly enough seen in the others. (7) In some few cases Albite 
twins may be twinned yet again, whence arise Compound Twins. 
Lastly, (8) Albite twins, simple or compound, may group together 
into either Parallel Growths, in which some face may be developed 
either so as to be coplanar throughout the entire group, or else 
into more or less Irregular Aggregates, in which any such coplanar 
growth of a particular face does not take place. 
One remarkable feature connected with these aggregates is the 
tendency manifested by many of them to arrange themselves in such 
a manner that further enlargement may, so to speak, facilitate the 
growth of the whole colony into one large crystal. This curious 
fact, which may almost be regarded as foreshadowing future events 
