Fig. 19.—Two intersecting parallel series of fractures produced upon each free surface of a prismatic block of stiff molders’ wax when broken by compression from the ends (after Daubrée and Tresca).
In contrast, the near-surface zone is called the zone of fracture. But different rocks possess different strengths, and these are subject to modifications from other conditions, such, for example, as the proximity of an uncooled magma. The zone of flow is therefore joined to the zone of fracture, not upon a definite surface, but in an intermediate zone described as the zone of fracture and flow.
Experiments which illustrate the fracture and flow of solid bodies.—A prismatic block prepared from stiff molders’ wax, if crushed between the jaws of a testing machine, yields a system of intersecting fractures which are perpendicular to the free surfaces of the block and take two directions each inclined by half of a right angle to the direction of compression ([Fig. 19]). This experiment may illustrate the manner in which fractures are produced by the compression within the zone of fracture of the lithosphere, as its core continues to contract.
To reproduce the conditions within the zone of flow, it will be necessary to load the lateral surfaces of the block instead of leaving them unconstrained as in the above-described experiment. The experiment is best devised as in [Fig. 20]. Here a series of layers having varying degrees of rigidity is prepared from beeswax as a base, either stiffened by admixture of varying proportions of plaster of Paris, or weakened by the use of Venice turpentine. Such a series of layers may represent rocks of as widely different characters as limestone and shale. The load which is to represent superincumbent rock is supplied in the experiment by a deep layer of shot.
Fig. 20.—Apparatus to illustrate the folding of strata within the zone of flow (after Willis).
When compression is applied to the layers from the ends, these normally solid materials, instead of fracturing, are bent into a series of folds. The stiffer, or more competent, layers are found to be less contorted than are the weaker layers, particularly if the latter have been protected under an arch of the more competent layer (pl. 2 A).
The arches and troughs of the folded strata.—Every series of folds is made up of alternating arches and troughs. The arches of the strata the geologist calls anticlines or anticlinal folds, and the troughs he calls synclines or synclinal folds ([Fig. 21]). When a stratum is merely dropped in a bend to a lower level without producing a complete arch or a complete trough, this half fold is termed a monocline.
