458 STUDIES FOR STUDENTS 
I therefore conclude from analysis, from experiments upon 
viscous and plastic bodies, from observations in the field, and 
from studies with the microscope, that I am justified in the state- 
ment that “he secondary structure of a rock which is deformed by 
plastic flow develops in the plane normal to the greatest pressure, and 
that this structure 1s true cleavage. 
We thus see that in both the case of pure shortening and 
the case of shortening combined with rotation the secondary 
structure is parallel to the greatest dimensions of the mineral 
particles. This parallelism may be macroscopically observed 
at very numerous localities in which schist-conglomerates occur. 
Some of the more important of these are the Hastings district 
of Ontario, Green Mountains of Vermont (Fig. 4), Felch Moun- 
tain district of Michigan, many localities in the basal conglom- 
erates of both the Upper Huronian and Lower Huronian in the 
Marquette district of Michigan, the Black Hills of Dakota, and 
the Front Range of Colorado. 
In different localities the degree of flattening of the pebbles 
varies from a small amount to that in which the pebbles are 
tions. In most cases where a solid mineral particle or an aggregate of mineral par- 
ticles having an individuality is much flattened, we have evidence of the very complicated 
shearing along many intersecting planes. The deformation, if carried far enough, 
ordinarily does not produce a single, or even two structures, but complete granulation. 
The sum total of the sliding along all of the shearing planes is to shorten the diam- 
eter of any given area in the direction of greatest pressure and to elongate it at right 
angles to this. During this deformation a given mineral particle may have become a 
multitude of mineral particles. However, the multitude in the aggregate differs in 
composition and consequently in strength from the adjacent similarly flattened areas. 
Moreover, many of the newly formed individual particles have similar forms in like 
positions. Oftentimes the readiest cleavage of many of the particles accords with 
their greater diameters. The result is that there is an easy parting or cleavage along 
the greater dimensions of the flattened areas and the greater dimensions of the newly 
developed particles. Even if the material is absolutely homogeneous so far as we 
can discover, the same principle applies, as shown by Tyndall for wax. Analogous to 
this is the case of stretched or compressed viscous Canada balsam, which takes on a 
structure not in the shearing planes, but parallel to the tension or at right angles to 
the pressure, as shown by polarized light. As shown by Professor Hoskins, the case 
of simple shearing does not differ from that of pure shortening except in that the mass 
as a whole is rotated (Fig. 3). If the above is true, it is clear that all combinations 
of pure shortening and of simple shearing in the zone of flow result in producing a 
cleavage which develops normal to the greatest pressure. 
