456 SLUDIES FOR STUDENTS 
in Massachusetts. Similar phenomena have been described in 
other countries at localities too numerous to mention. In all of 
these cases there can be no doubt that the magmas were under 
hydrostatic conditions and transmitted their pressures at every 
point normal to the rocks with which they were in contact. 
There would therefore seem to be no escape from the conclusion 
that in such cases the secondary structure developed in planes 
normal to the pressure. 
In homogeneous slates having cleavage it is not usually pos- 
sible to follow the particles through their movements to their 
final positions. However, in rocks which are not so nearly homo- 
geneous there are numerous minor plications. Where these pli- 
cations are arranged in a symmetrical fashion, that is, do not 
have one set of limbs longer than the others, they give strong 
evidence that the thrusts were in opposite directions and normal 
to the secondary structure. If the rock were homogeneous the 
crumplings would disappear and the movement of the particles 
would more nearly follow the law of plastic flow. Such defor- 
mation of the particles may be without differential movement 
normal to the pressure, and is illustrated by Figs. 1 and 2 in an 
ideal case. It is not necessary that the particles be of the same 
size or of the same strength. As is known by macroscopical 
study of the schist-conglomerates and by microscopical study 
of ordinary slates and schists, if the movement continues far 
enough the old mineral particles are flattened, or flattened and 
rotated, into parallel positions. New minerals develop with sim- 
ilar orientation. Therefore, as a consequence of the flowage of 
the stratum, the induced arrangement of the particles necessa- 
rily produces a capacity to cleave in the plane of the two longer 
axes of the mineral particles, as in this direction the rock may 
part between them. 
In the case of forces resulting in rotatory movement, the 
structure may be explained as developing in the normal planes 
as in the case of pure shortening. The direction of shortening 
of any small area varies in its relation to the component par- 
ticles at each successive moment. This variation may be due 
