DEFORMATION OF ROCKS 463 
perpendicular to the fibrous structure and by less pressure in the 
direction parallel to the fibers. This results in shortening in all 
radial directions and elongation at right angles to this, and thus 
the fibrous structure is formed normal to the plane composed of 
the radial direction of greater pressure. 
It has been seen on a previous page that unmodified igneous 
rocks may have the capacity to cleave, and that the structure is 
similar to a secondary structure in sedimentary rocks. There is 
not so great a difference between the two in this respect as might 
at first be supposed. The arrangement of the mineral particles 
in the igneous rocks is caused by the similar original crystalline 
orientation of the mineral particles produced by unequal stresses 
in three dimensions and by the rotation of flowage. In the sed- 
imentary rock the same is true, but the flatness of the particles 
is due to another cause. The chief difference is, however that 
the lava is a viscous liquid and the crystallized rock a plastic 
solid. The manner in which the parallel minute mineral particles 
which produce cleavage in igneous rocks change their direction 
in passing around a large porphyritic crystal is as similar as 
possible to the way in which laminar mica scales ina slate or 
schist change in direction in passing around a large refractory 
grain. In both cases the material wraps around the rigid par- 
ticles which were sufficiently strong to partially or wholly resist 
deformation. 
The above explanation of cleavage makes this structure also 
analogous to the capacity to part along bedding planes in sedi- 
mentary rocks. As each erosion particle has unequal diameters, 
it comes to rest in most cases with its longer diameters in the 
plane of bedding, thus giving at the outset a laminated structure. 
This structure may be emphasized by the pressure of gravity. It 
therefore follows that the unaltered sedimentary rock ruptures 
more readily along its bedding plane than elsewhere. That the 
capacity to part parallel to bedding is less marked than parallel 
to the cleavage of slates and schists is due to the fact that the 
ratio of the minor to the mean and major diameters of the min- 
eral particles is not so large and the regularity of the parallel 
