DEFORMATION OF ROCKS 459 
transformed into leaf-like areas. Everywhere and in all grades 
of change the secondary structure accords with the longer diam- 
eters of the flattened pebbles. The some phenomena have been 
observed by me in microscopical studies upon fragmental par- 
ticles in hundreds of sections from the semicrystalline formations 
from many parts of America. 
According to Becker’s explanation of slaty cleavage, there is 
FIG. 4 
Fig. 4. Schist conglomerate, showing pebbled character when cut transverse to 
the major direction of shortening, and gneissoid character when cut in other direc- 
tions, from Plymouth, Vt., after Hitchcock. 
an important discrepancy between the direction of the structure 
and the greater diameters of the flattened mineral particles." 
If the above observations are correct, it would seem that 
there is no escape from the conclusion that the secondary 
structure which I here describe as cleavage is not developed in 
the manner described by Becker and cannot be explained by his 
theory. I therefore return to the old explanation of the English 
geologists—which perfectly accords with the facts—that this 
secondary structure develops in planes normal to the pressure. 
In further confirmation of this explanation are certain phe- 
nomena described to me by Diller and Keith. 
Diller makes the following statement: 
«Finite Homogeneous Strain, Flow and Rupture of Rocks, GEORGE F, BECKER, 
Bull. Geol. Soc. Am., Vol. IV, pp. 55-66. 
