Reading References for Chapter IV
J. S. Diller. The Educational Series of Rock Specimens collected and distributed by the United States Geological Survey, Bull. 150 U. S. Geol. Surv., 1898, pp. 1-400.
L. V. Pirsson. Rocks and Rock Minerals. Wiley, New York, 1908.
Sir John Murray. Deep-sea Deposits, Reports of the Challenger expedition, Chapter iii.
L. W. Collet. Les dépôts marins. Doin, Paris, 1907 (Encyclopédie Scientifique).
CHAPTER V
CONTORTIONS OF THE STRATA WITHIN THE ZONE OF FLOW
The zones of fracture and flow.—It is easy to think of the atmosphere and the hydrosphere as each sustaining at any point the load of the superincumbent material. At the sea level the weight of air upon each square inch of surface is about fifteen pounds, whereas upon the floor of the hydrosphere in the more profound deeps the load upon the square inch must be measured in tons. Near the lithosphere surface the rocks support by their strength the load of rock above them, but at greater depths they are unable to do this, for the load bears upon each portion of the rock with a pressure equivalent to the weight of a rock column which extends upward to the surface. The average specific gravity of rock is 2.7, and it is thus easy to calculate the length of the inch square column which has a weight equivalent to the crushing strength of any given rock. At the depth represented by the length of such a column, rocks cannot yield to pressure by fracture, for the opening of a crack implies that the rock upon either side is strong enough to prevent the walls from closing. At this depth, rock must therefore yield to pressure not by fracture, as it would at the surface, but by flow after the manner of a liquid; and so the zone below this critical level is referred to as the zone of flow.