THE ORIGIN OF FOLIATED CRYSTALLINE ROCKS 241 
To the writer, the conception of differences in molecular attrac- 
tion within and across the cleavage of a mineral seems to afford a 
plausible explanation why the elongation of minerals under unfavor- 
able conditions for growth takes place parallel to the plane of 
mineral cleavage. This idea, though, is of course purely specula- 
tive. There may be also reasons, not at present known, which 
would cause minerals, such as biotite, to be more stable under 
conditions of differential pressure when lying with their planes of 
cleavage parallel to the greatest pressure. It is almost useless to 
conjecture what causes the normal elongation of some minerals at 
right angles to their cleavage, though one might suggest molecular 
form or arrangement as possibilities. 
Inferences regarding the texture of primary gneisses from mineral 
elongation.—Descriptions of primary gneisses seem to show that 
a more or less parallel arrangement of platy mineral constituents 
is a constant characteristic of this rock type. This arrangement 
may be considered to have been caused by (1) rotation of minerals 
in a still fluid magma, by (2) development of abnormal elongated 
mineral forms during crystallization, by (3) parallel growth of 
minerals with normal form development, or by (4) granulation, 
slicing, or gliding. The first method would probably give rise to 
characteristic igneous textures. Microscopic studies of schists 
indicate that the last three processes would tend to the production 
of the metamorphic type of texture. 
The writer has endeavored to point out in preceding sections 
that the development of abnormal elongated mineral forms is an 
important feature in the production of schistosity in metamorphic 
rocks. The following review c* the most abundant minerals of 
primary gneisses is intended to show that this process is also a 
leading one in the crystallization of primary gneisses and, accord- 
ingly, that the texture of such rocks must have some of the 
characteristics of the metamorphic type. 
Among the minerals most frequently mentioned as having been 
rotated in a magma is biotite.1 Fhe following points seem to indi- 
cate, however, that in the case of this mineral rotation must be of 
secondary importance and that its orientation is largely due to the 
1 E.g., Geol. Mag., N.S. (Decade 4), IV (18097), 348. 
