March 4, 1921] 



SCIENCE 



203 



support considerable structures against iso- 

 static tendencies; that it is not essentially 

 jnolten or fluidal in the ordinary sense; that 

 piolten magmas are probably local and inci- 

 dental. 



As to depth and distribution of the move- 

 ments, and as to the manner of movement, 

 whether by fracture or plastic flow or by some 

 unknown process, there is wide divergence of 

 opinion. Likewise, there is doubt as to the 

 laws or control under which stresses may be 

 transmitted. We may refer briefly to these 

 questions. 



Does a Zone of Weakness or Mohility Exist 

 in the Unseen Depth? — A common conception 

 of the distribution of movement deep below our 

 ^one of observation confines it to a single 

 jspherical zone of weakness or mobility sur- 

 rounding the centrosphere and surrounded in 

 turn by a rigid shell. This zone is supposed 

 to be ma,rked by a capacity to yield readily to 

 long enduring strains. It may be in part the 

 generating zone of magmas, which may be a 

 factor in its supposed easy yielding. The con- 

 ception of the existence of a weak and mobile 

 zone has found expression in several ways. 

 , The widely held belief in the existence of a 

 zone of rock flowage below a surficial zone of 

 fracture has commonly carried with it an as- 

 sumption of the relative weakness and mobil- 

 ity of this zone. In fact " zone of rock flow- 

 age " and " zone of weakness " have come to be 

 ^Imost synonymous in discussion of this prob- 

 lem. Doubt as to this correlation is expressed 

 later. Even if the existence of a single zone 

 (of rock flowage were proved, it does not neces- 

 sarily follow that this is a zone of weakness. 



Van Hise assigned a depth of only six miles 

 to the top of this zone, though with the im- 

 portant reservation that increased rigidity 

 under containing pressures would greatly in- 

 crease this figure. 



, Adams and Bancroft,' on the basis of ex- 

 periments with rock failure under great con- 

 taining pressures, conclude that the amount of 

 jtangential thrust required to produce move- 

 pients increases so rapidly below the surface 



8 ioc. dt., p. 635. 



" that the great movements of adjustment by 

 jTock flow or transference of material in the 

 earth's crust from one point to another — other 

 than the transference of rock in a molten con- 

 dition — must take place comparatively near 

 the surface," and that the ease of movement 

 " increases rapidly in proportion to their near- 

 ness to the surface." The mobile zone thus im- 

 plied is inferred from experimental results to 

 be limited to depths within 35 miles, below 

 which a condition of no mobility seems to be 

 assumed. 



Gilbert conceived " a relatively mobile layer 

 separating a less mobile layer above from a 

 nearly immobile nucleus," the mobile layer 

 agreeing in depth with the depth of isostatic 

 compensation. 



, Barrell called this weak zone the astheno- 

 sphere and assigned its provisional boundaries 

 at depths of Y5 and 800 miles from the surface. 

 iThis he conceived to underlie the zone of iso- 

 /static compensation, which was calculated by 

 jHayford to be 75 miles below the surface. 

 ) Hayford assumed concentration of move- 

 pient within the lower part of the zone of iso- 

 static compensation, that is within Y5 miles 

 of the surface. 



i Willis concludes tha:t there is a zone of ad- 

 justment below 40 miles and extending to the 

 {base of the asthenosphere, and that the ad- 

 justments necessary to isostatic undertow take 

 jplace mainly between 45 and 100 miles from 

 the surface. 



, In contrast to these conceptions of a deep 

 mobile zone, are the views of T. C. Chamberlin 

 and R. T. Chamberlin, who postulate multiplic- 

 ity and irregularity of movement zones. 

 ; R. T. Chamberlin' concludes that mountain 

 jmaking diastrophism affects wedge shaped 

 piasses and implies steeply inclined zones of 

 piovement. 



T. C. Chamberlin emphasizes the superficial 

 nature of diastrophic movements of the moun- 

 ;tain making kind, whether these are tangen- 

 jtially compressive or the result of creep of 

 continental masses under gravity. In regard 

 ,to deeper, so-called massive, movements of the 



, » Loc. oit. 



