DEFORMATION OF ROCKS 461 
parts of the rock in which resistant particles are absent. 
Because the structure of the cleavage, where hard particles are 
present, simulates a mesh structure, loose observation might lead 
to the conclusion that there are present two intersecting struc- 
tures. The prime characteristic of a true mesh structure is that 
two diagonal structures shall intersect. As shown subsequently 
(p. 465), interesting diagonal fissility may develop in the shear- 
ing planes. 
The deviation of cleavage about hard particles is explained 
by the fact that the more resistant grains act as transmitters 
of forces. At any given point at the exterior of a hard particle 
the direction of greatest pressure is normal to the grain, and 
hence the peripheral arrangement of the cleavage about the grain. 
The principle is precisely the same as that explaining the develop- 
ment of zonal cleavage about intrusive batholites. (See pp. 
455-456.) 
The crucial point in deciding whether there is a structure 
which develops in the normal plane is the simple matter of fact 
as to whether or not the flattening of the original particles cor- 
responds with this structure. In testing this point it is best to take 
examples in which the flattening of the particles is not too great 
(see Fig. 4), else the lessening discrepancy between a possible 
structure called cleavage by Becker,’ developing in the planes of 
“maximum tangential strain,’ and that regarded as cleavage by 
me, developing in the normal planes, may be overlooked. As 
already explained, I hold that the facts of the field accord with 
my position. Upon this crucial point of fact I ask the obser- 
vation of geologists, for by the facts of occurrence must be 
judged the adequacy of the explanation offered as to the man- 
ner of action of the forces which produce cleavage. It is 
believed by me, as will be seen, that the theory that cleavage 
is developing at any given moment normal to the pressure fully 
explains all the diverse facts of true cleavage in both homoge- 
neous and heterogeneous rocks. 
Fin te Homogeneous Strain, Flow and Rupture of Rocks, GEoRGE F. BECKER, 
Bull. Geol. Soc: Am., Vol. IV, pp. 55-66. 
