OROGENY AND EARTH’S ROTATION 
69 
there must be molar movement strictly analogous to rock-flow in 
the depths. It follows that hard limestone bands intercalated in 
thick shale beds in such situations as in the Appalachians, are 
capable of pronounced and continuous bending without rupture 
and readjustment of parts. 
Whether, then, there be rock-flow or rock-fracture in a given 
crustal prism depends primarily upon the specific character of the 
stress imposed, and secondarily upon the localization of the strain 
effects and the cohesion of the rock-mass. In the majority of 
cases the time-element is sufficiently protracted to allow of molar 
movement without mass rupture. 
As commonly used, the term diastrophism designates any upris¬ 
ing or down-sinking of the earth’s crust, as if such passive move¬ 
ment were in itself an initial force instead of mere tectonic adjust¬ 
ment. It is, perhaps, not always possible to resolve readily such 
crustal movement into its component forces, or to reduce it to its 
prime factors of gravitation, tangential tension, and hydrostatic 
pressure. The movement is usually measured by the relative posi¬ 
tion of the sea-level and is made primarily a function of a con¬ 
tracting nucleus. 
Determination whether a given diastrophic movement is oro- 
genic or epeirogenic in character is necessarily vague. Shaler 
expresses the belief that continents represent contractions of the 
whole earth’s crust, whereas mountain systems are to be consid¬ 
ered merely as foldings of the more superficial layers of the litho¬ 
sphere. With this opinion Suess readily agrees. This view is 
almost a necessary consequence of the contractional hypothesis, 
rather than a deduction from observation. It is, however, doubt¬ 
ful whether the distinction is a logical one, even according to the 
contractional principle. Experimentally the same release of tan¬ 
gential stress gives rise to both continental arches and orographic 
foldings. So that the same general influences account for both. 
Continental, regional and local corrugations thus appear to be the 
immediate accommodation of the greater telluric strains. 
In its larger tectonic aspects diastrophism becomes a negligible 
quantity. It thus mainly has to do not with tectonics but with strat¬ 
igraphy. In the latter role its real function is principally taxo¬ 
nomic. As such it becomes the foundation of terranal classifica¬ 
tion and nothing more. 
