IMPLICATIONS OF COMPLETE ISOSTASY 305 



eiit writer will return to this point) ; otherwise excesses or defects of 

 mass in prisms of considerable area could be upheld or held down by the 

 material strength of the shell when it is supported, as it is, upon a con- 

 tained spherical continuum of subjacent matter (whatever the condition 

 of this), just as it can, and does, support small loads over small areas 

 and, perhaps, large loads over very small areas. But, on the other hand, 

 in geology there are many observed facts which indicate that the earth's 

 outer shell is very strong, especially in its upper portion — at least in 

 sustaining, transmitting, or in offering resistance to stresses exerted in 

 the horizontal direction. Many of the effects observed and ascribed to 

 horizontal stresses are on the grand scale, and the intimate responses ex- 

 pectable in weak materials do not seem often to occur. 



Is there need here to recite the observed facts of mountain folding, 

 overthrusting (or under dragging), and other such evidences of horizontal 

 shortening and reduction of area presented in so many places on the 

 regional if not on the world-wide scale ? While we must, of course, scru- 

 tinize from time to time the explanations of this action in respect to 

 mechanisms and details, is there anything known at present to indicate 

 any adequate explanation of the grander dynamical phenomena except 

 horizontal compression in fairly competent materials, despite the objec- 

 tions to this so far raised? If so, the present writer is not aware of it. 

 And any modifications of opinion in regard to details which seem now to 

 deserve consideration still appear to require an outer shell strong with 

 respect to forces exerted horizontally. Moreover, the probability of this 

 property of great strength in the shell is sustained by appeal to certain 

 physical facts, like the effects found when great pressures are applied to 

 rock substances confined in strong, rigid containers, as in the experiments 

 conducted by Adams 8 and others. Here, however, the factor of time — 

 that quantity so abundantly available in geologic happenings — is neg- 

 lected. And the importance of the factor of time is very great. 



Moreover, to confuse us, there are also observed geologic facts which 

 indicate that the outer shell is weak (perhaps, however, weak especially 

 in respect to stresses productive of motion in the vertical direction, as 

 Leith has so clearly pointed out). And, again, appeal may be made to 

 certain physical principles and experiments to support this, as Hayford 9 

 indicated so emphatically a few years ago. To illustrate, mention may 

 be made of the slow, progressive, permanent yielding (whatever its na- 

 ture) which some substances ordinarily rigid undergo under enduring 

 stresses near to their elastic limit (even in some instances well below their 

 elastic limit for so-called instantaneous forces), of the predominance of 



XX— Bull. Geol. Soc. Am., Vol. 33, 1921 



