202 



SCIENCE 



[N. S. Vol. LIII. No. 1366 



it is known ttat tlie horizontal distribution of 

 densities is heterogeneous. The density is low 

 in the earth protuberances and high in earth 

 depressions, as if the earth masses were in 

 flotational equilibrium. The subcrustal densi- 

 ties are balanced against topographic relief. 

 This is called isostatic equilibrium. Certain 

 parts of the earth, called negative elements by 

 Willis,' seem to have been subjected during 

 geologic history to long-continued deposition. 

 Other parts, called positive elements, have been 

 jnore commonly subjected to erosion than 

 deposition. Negative elements are heavy and 

 positive elements are light. Loading and un- 

 Joading is not necessarily the primary cause of 

 movement, but may serve to accentuate an in- 

 Jierent and prevailing tendency to isostatic ad- 

 justment between masses of differing density. 

 Isostatic balance is not complete. Some 

 parts of the earth vary from this condition, 

 suggesting that they have sufficient strength to 

 sustain themselves in opposition to isostatic 

 tendencies. 



, The observed relations between density and 

 relief may be explained on the assumption that 

 the differences in density extend uniformly to 

 a depth of about 75 miles, called the depth of 

 isostatic compensation. This figure is favored 

 by geodesists. No one knows, however, the den- 

 sity gradients deep below the surface, or the 

 extent to which there is heterogeneous vertical 

 distribution of density. If instead of assum- 

 ing the uniform downward extension of densi- 

 ties observed at the surface, assumptions are 

 made of other vertical distributions of density, 

 various other depths of compensation may be 

 calculated, ranging up to several hundred 

 ;miles. So far as geologic evidence goes, it 

 seems to favor the view that depth of compen- 

 sation is not uniform. 



I A comparison of the up-lift of mountain 

 masses with their horizontal shortening indi- 

 cates how deep the mountain making move- 

 m.ents have extended.* In general sharp close 

 Willis, Bailey, ' ' Discoidal Structure of the 

 Lithosphere, " Bull. Geol. Soo. of Am., Vol. 31, 

 1920, p. 277. 



6 Chamberlin, E. T., "The Appalachian Folds of 

 Central Pennsylvania," Jour. Geol., Vol. 18, 1910, 



folding indicates a comparatively shallow 

 depth, whereas broad open folds, approaching 

 the plateau type of deformation, can be ex- 

 plained only by movements of material extend- 

 ;ing to great depths. Major features of con- 

 tinental and oceanic relief also seem to require 

 the latter inference. If the amount of short- 

 ,ening observed in some mountain ranges were 

 to extend downward indefinitely, mountains 

 piuch higher than those actually formed would 

 have resulted; hence the conception of consid- 

 erable movements of a shallow shell without 

 equivalent movement below, and thus perhaps 

 the conception of mobility of an intervening 

 Jayer, though at widely different depths in dif- 

 ;ferent localities. 



I Geologic evidence points to periodicity in 

 earth movements, indicating that the adjust- 

 pient to stress is not uniform and continuous. 

 I Finally, magmas originate well below our 

 zone of observation and presumably take part 

 in the mechanical easements. From the known 

 conditions of rigidity already indicated, it 

 seems certain that liquid condition is local and 

 intermittent. Quoting from Gilbert i'' 



The continuous or secular relations of pressure, 

 temperature, and density in the subterranean re- 

 gion from which liquid rock rises at intervals may 

 be assumed to be such that moderate change of 

 condition either induces liquefaction or else so 

 lowers the density of rock already liquid as to 

 render it eruptible; and such a balancing of con- 

 ditions implies some sort of mobility. 



j From these facts it is clear that earth move- 

 ments extend to considerable depths below our 

 zone of observation, that the movements are 

 periodic, that the earth as a whole is more 

 rigid than steel when subjected to sudden 

 stresses like earthquake shocks or tidal pulls, 

 that it yields slowly and periodically to long 

 continued stress; that as a whole it is suffi- 

 ciently weak to allow a large measure of iso- 

 static adjustment, but still strong enough to 



pp. 228-251 ; ' ' The Building of the Colorado Rock- 

 ies," Jour. Geol., Vol. 27, 1919, pp. 145-164, 225- 

 251. 



7 Gilbert, G. K., ' ' Interpretation of Anomalies 

 of Gravity," Prof. Paper 85, TJ. S. Geol. Survey, 

 1914, p. 34. 



