DEFORMATION OF ROCKS 457 
to the work of stress-couples producing simple shearing, to a 
change in the direction of pressure, or to a rotation of the area 
concerned, or two or all combined. The final result, as shown by 
Professor Hoskins, is to deform a given homogeneous area as 
though it were shortened in but a single direction, and after this 
was rotated (Fig. 3). The cleavage is at right angles to the 
direction of greatest shortening of the area in its final position, 
Fic. 3.—A portion of a bed ABCD, containing round pebbles, is shearing into 
the form 4'4'C'D'. The cleavage is parallel to &'’.S’. The result is the same as 
through the material had been flattened by pressure along PQ, and the mass after- 
wards rotated until AS fell in line with A’.S’. After Hoskins. 
this structure being due to the capacity to part parallel to the 
greater dimensions of the mineral particles. This resultant posi- 
tion is not normal to the final direction of greatest pressure, 
but at any given moment the deformation occurring is itself 
normal to the pressure. Newly developing minerals also tend 
to form at any moment with their shortest axes in the direction 
of pressure, and are rotated by the simple shearing exactly the 
same as the original minerals. Therefore the shortest axes of 
both new and old minerals are in the same direction. Thus the 
cleavage develops strictly in the normal planes, but its position 
by the rotation of simple shearing is inclined to the final direc- 
tion of pressure.’ 
tIt is admitted to be somewhat difficult to understand the precise nature of the 
ultimate interior movements by which pure shortening involving shearing along two sets 
of intersecting diagonal planes produces a structure normal to the greatest pressure. How- 
ever, Professor Hoskins shows that during pure shortening it is only for a moment that 
any given plane is one of maximum tangential stress and pure sliding, and that all planes 
inclined to the greatest pressure are shearing planes (Figs. 1 and 2). If some struct- 
ure should be produced along the shearing planes in the zone of flow, one would not 
expect a structure in a single direction, but structures in an indefinite number of direc- 
