446 GEORGE F. BECKER 
nate heating and cooling is due to the flow on such planes.* 
Several mineralogists have also come to the conclusion that the 
twinning of minerals in nature is largely due to the strain they 
have undergone. This idea appears to have been suggested in 
the first instance by Mr. Max Baur in 1878.7. In massive rocks 
the minerals are usually strained as a result of cooling and there 
is much evidence from independent observers3 that the polysyn- 
thetic structures of feldspars and pyroxenes in rocks are wholly 
or in part due to these strains. 
In view of the evidence merely outlined above it appears to 
me utterly impossible to deny that solid flow does as a matter of 
fact induce a true cleavage which is parallel to the lines of relative 
tangential motion or gliding, this cleavage not necessarily being 
~accompanied by any actual ruptures however microscopic. It 
appears also that this action goes on in thoroughly homogeneous 
substances such as calcite and quartz. These minerals are not 
indeed amorphous, but that fact only modifies the direction in 
which flow will occur most readily, not the principles governing 
flow. The schistosity of deformed quartz, calcite, feldspar and 
rock salt crystals cannot possibly depend on the flattening or 
rotation of included particles. 
The theory of slaty cleavage held by most geologists ascribes 
the structure to the flattening of particles at right angles to the 
line of pressure and the rotation of mica scales towards the same 
position. There are some objections to this.view. In the first 
place only the exceptional irrotational strains produce flattening 
at right angles to the line of force so that, even if fissility were 
produced by flattening, it would be a mistake to infer that the 
direction of force was normal to the cleavage.4 In the second 
place this theory assumes either that the flattened particles resist 
fracture more persistently than the matrix in which they are 
*Loid., Vol. 8, 1888, p. 1 and Vol. 10, 1892, p. 123. 
2 Zeitsch. der D. Geol. Ges., Vol. 30, 1878, p. 323. 
30. Mugge, L. van Werweke, Judd, etc. 
‘In all rotational strains the inclination of the greatest axis of the ellipsoid varies 
with the amount of strain. It therefore changes during strain when the direction of 
the force is fixed. 
