202 SLODIES FOR STUDENTS 
well-nigh 10,000 meters of material, although this is difficult to 
demonstrate. 
Whether rocks flow or fracture is in many cases largely 
dependent on the rapidity of deformation. As pointed out by 
Professor Hoskins, a rock under a certain pressure in two 
directions, when rapidly deformed by a greater pressure in a 
third direction, may be fractured, and when less rapidly deformed 
may flow. This results from the fact that the elastic limit of a 
rock is always less than its ultimate strength. During the time 
of rapid deformation the rock may be fractured and crevices 
and cracks formed which are subsequently closed by plastic flow 
even if the stresses decrease in amount. Also, as the stresses 
were slowly increasing, there may have been very considerable 
flowage before any fractures were produced. Hence, even in 
homogeneous rocks, the zone of fracture and the zone of flowage 
are not sharply separated from each other, and the upper part of 
the zone of flowage is at different depths under varying condi- 
tions of stress. This principle is illustrated by the distortion of 
rocks in ancient buildings and by slabs of marble suspended by 
their ends in cemeteries. This latter case shows how important 
the element of time is in the deformation of rocks, and that, 
given a sufficient time, a stress much below the ultimate strength 
may surpass the elastic limit and result in flowage. It thus 
becomes clear that there may be a very considerable thickness 
for any given rock in which it may be in the zone of fracture 
or in the zone of flowage, depending upon the amount of 
differential stress. 
(3) Rocks buried to such a depth that the weight of the superin- 
cumbent strata exceeds their ultimate strength are in the zone of plas- 
ticity and flowage. These are the conditions of folding, for per- 
manent perfect flexure is possible only by flowage of material. 
It is a contradiction to suppose that cracks and crevices can 
form under these conditions. Were it possible for an opening 
to be made in any way, under the hypothesis the rock would 
*An Illustration of the Flexibility of Limestone, by ARTHUR WINSLOW. Am. 
Jour. Sci. (3), Vol. XLIII., 1892, pp. 133, 134. 
