226 Collie—Geology of Conanicut Island, R. I. 
selves to pressure, hence they fold frequently and uniformly. 
The dark schists with their quartzuse layers do not so readily 
conform to pressure and longer resist it; hence when pressure 
accumulates as it must until the rock no longer can resist, 
they form into folds, large because of the accumulated pressure 
which causes them, lacking the fine even characteristics of seri- 
cite folds because of the coarseness of the materials and their 
inadaptibility to move over each other or readily adjust them¬ 
selves to pressure. Augen are frequently found in the black 
schist and are sometimes folded, indicating, as the phenomenon 
of the folded schistosity indicates, that the master pressure 
which induced the schistose character of the rocks was first in 
point of time and that subsequently another force has slightly 
folded the schistosity and the augen. The minute puckerings are 
frequently so compressed that in the axes of the crests incipient 
fractures appear, the beginning of another set of schistose 
planes. 
In some portions of the rock this new set is plainly indi¬ 
cated and in these localities there is true double schistosity. 
This second set of cleavage planes is, as already indicated, the 
slip-strain cleavage of Sorby. The planes are very numerous, 
frequently one thousand to the inch and are unquestionably gliding 
planes along which movement takes place, when crushing is so 
great that folding is no longer possible. When gliding planes 
are so numerous as that, a very distinct schistosity is observ¬ 
able microscopically. Where the planes are few, no such struct¬ 
ure is observable to the unaided eye. 
Schistosity is a matter of degrees. A rock is truly schistose 
when the planes are few and are not distinguishable; only when 
the gliding planes are numerous and closely appressed does it 
becomes a macroscopic structure. That the sericite flakes of 
these schists are quite largely developed by pressure is indi¬ 
cated by the method of formation. Initially they begin to form 
in planes of faulting or planes of movement. In rocks which 
have suffered little deformation the sericite may be largely con¬ 
fined to these planes. If pressure continues however and the 
movement of particles in the rock becomes universal, sericite 
forms throughout the mass of the rock. Rarely some peculiar 
