SCH IS LOSE SLA TY CLEAVAGE 445 
rail heads are rendered schistose near railway stations by the 
arrest of moving trains would show the action. In drawing wire 
similar phenomena appear and in both cases the direction of 
cleavage is that of flow. Experiments on semi-solids such as 
pastry and clay are less satisfactory inasmuch as the presence 
of fluids must disturb the purity of the results, yet in so far as 
their behavior differs from the known behavior of true fluids, 
they are instructive. Such experiments when carefully scruti- 
nized yield results compatible with the theory of this paper. 
That flow with attendant weakening of cohesion is the origin 
of slaty cleavage appears to have been recognized by the first 
investigator to offer a mechanical explanation of this structure. 
John Phillips in 1843 ascribed cleavage toa ‘creeping movement 
among the particles of the rock, the effect of which was to roll 
them forward.” Mr. Daubrée says that schistose or laminated 
structure is a direct consequence of gliding (glissement), a term 
which he explains by the remark that the different velocities 
acquired by contiguous molecules make them glide past one 
another. Actual cases are on record in which evidences of 
diminished cohesion (without rupture) make their appearance in 
rocks in directions parallel to faulted joints. Professor Judd has 
described such,? and they clearly show that flow takes place asa 
preliminary to jointing and in the less strained portions of jointed 
rocks. 
d 
The deformation of crystals on “gliding planes” which is 
usually accompanied by secondary twinning is a case of flow in 
eolotropic masses. The gliding planes also become after relative 
movement planes of easy cleavage, not in general identical in 
direction with the inherent cleavage planes of undeformed 
crystals. The gliding planes in the case of calcite seem to have 
been known to Huyghens and they have been studied in a great 
many minerals during the last few decades. Professor Judd has 
produced gliding planes in quartz by means of pressure, and it is 
probable that the cleavage which is produced in quartz by alter- 
« Bull. Soc. Géol. de France, Vol. 4, 1876, p. 541. 
2 Mineralogical Mag., Vol. 7, 1886, p. 81. 
