204 



SCIENCE 



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



kind reflected in major features of continental 

 jand oceanic relief, he does not assume any 

 jnobile substratum, but rather steeply inclined 

 (Zones of movement. As he states it :^° " In- 

 herited inequalities of specific gravity are, 

 perhaps more than any other agency, the gov- 

 erning power in shaping if not actuating dias- 

 itrophic movements " — but that " the normal 

 jnode of isostatic adjustment in such an earth 

 is thought to be wedging action in the form of 

 naovements on the part of its constituent ta- 

 pering prisms, conical, pyramidal, or otherwise, 

 in response to the varying stresses imposed on 

 ithem. . . . They should reach to whatever 

 ■depths may be seriously affected by differential 

 stresses of an order requiring readjustment. 

 No undertow in a hypothetical mobile sub- 

 stratum is necessarily involved and none is 

 postulated." 



These are only a few of the views that 

 iHiight be cited to indicate the wide range of 

 hypotheses possible as to depth, number, and 

 .attitude of deep mobile zones. The very di- 

 versity of these views emphasizes the restricted 

 irange of known facts. The requirement of 

 jproof naturally rests most heavily on hypoth- 

 eses which most precisely restrict the locus of 

 movement. So many assumptions must enter 

 into this proof that in our present state of 

 knowledge it can not be rigorous. The safest 

 scientific attitude for the time being would 

 seem to be one of rigid adherence to the known 

 facts, and the recognition of the possibility of 

 piore than one hypothesis to explain them. 

 This is not incompatible with a sympathetic 

 attitude toward the efforts of those attempting 

 proof of a single hypothesis. 

 I Until the time comes when it is possible to 

 furnish definite proof of any specific localiza- 

 tion of movement, my own inclination is to 

 keep clearly in mind the distribution of move- 

 jnents within the zone of observation, already 

 .summarized, as perhaps the best guide to the 

 pondition that may be assumed at least for 

 isome distance below our lowest observations. 

 |This measuring stick is short, but there are 



10 Ohamberlin, T. C, ' ' Diastrophism and the 

 rormative Processes," Jour. Geol., Vol. 21, 1913, 

 p. 520; Vol. 26, 1918, p. 197. 



^ome reasons for believing that it is as good as 

 jany yet available to measure our course 

 (through the complex of hypotheses possible in 

 (the deep zone. Especially is it desirable to 

 keep in mind the fact that cleavage, indieat- 

 |ing rock flowage, as observed in the deepest 

 (part of our zone of observation, does not in 

 (general have an attitude required by the con- 

 ception of tangential shearing in a mobile zone. 

 (This does not disprove a different attitude be- 

 low, but it does eliminate an affirmative bear- 

 ing on the question which has been sometimes 

 implied. 



Are Deep Movements Accomplished hy 

 Rock Flowage Bather than hy Roch Frac- 

 ture? — It remains to consider the manner or 

 processes through which deep movements are 

 accomplished, whether by plastic fiow, by 

 fracture or by some combination of these 

 kinds of deformation. The widely current 

 hypothesis is that deformation in the deep 

 zone is mainly by rock fiowage. The de- 

 formed rocks have not been seen, nor have 

 the environmental conditions been accurately 

 measured; yet there are weighty considera- 

 tions favoring this view: 



Experimental work has shown that rock 

 flowage requires containing pressures equal at 

 least to the crushing strength of rocks, and 

 these pressures surely exist in the deep zone. 

 Within the zone of observation even the 

 strongest rocks have locally suffered rock 

 flowage and hence have locally, even at that 

 shallow depth, been under containing pres- 

 sures sufficiently in excess of their crushing 

 strength to permit flowage. "With greatly in- 

 creased pressures at greater depths it is logical 

 to argue that conditions for flowage would be 

 improved. Under these conditions the resist- 

 ance to deformation is a fimction of the 

 internal friction or viscosity of the rock. 

 This property does not of necessity bear any 

 relation to the compressive strength or com- 

 petency of the rock — qualities which deter- 

 mine its behavior in the absence of great con- 

 taining pressiires. Quartzite or granite, so 

 far as we know, may have no greater viscosity 

 than marble or slate. Adams' experiments 

 show diabase and marble in a composite 



