260 



SCIENCE 



[N. S. Vol. XXIX. No. 737 



horizontal movement shown by folding seems 

 far in excess of the subcrustal creep in the 

 outer fifty or hundred miles of the earth's 

 crust needed to restore the isostatic adjust- 

 ment between the regions on the two sides of 

 the zone of folding. Furthermore, the vertical 

 readjustments which take place in epeirogenic 

 movements are not simultaneous in origin 

 with the horizontal movements. 



Somewhat similar difSculties seem to face 

 the larger hypothesis of Willis in which con- 

 tinental and oceanic segments are concerned 

 rather than adjacent geanticlines and geosjm- 

 clines. The average continental surface stands 

 about three miles above the average ocean 

 bottom, owing to the lighter subcontinental 

 matter. The isostatic compensation, as Hay- 

 ford has shown, is complete at a distance of 

 about seventy to a hundred miles from the 

 surfaces. A column of matter on the edge of 

 an oceanic segment and extending to this 

 depth will consequently have its top pressed 

 seaward by a pressure due to the greater 

 height of the continental mass, a pressure re- 

 sisted by the rigidity of the surface of the 

 oceanic segment. The foot of the column, 

 however, will not be strained in either direc- 

 tion, since the weights of the continental and 

 oceanic segments at this depth are equal. Any 

 intermediate point in the column will be 

 pressed seaward with an intermediate pressure. 



The initial cause of that horizontal move- 

 ment which is due to isostatic adjustment on 

 the continental margins would, therefore, be 

 an outward spreading of the continental mar- 

 gin by flowage and normal faulting. The 

 surface being lowered by this means, the sub- 

 continental pressures would be lessened and a 

 landward movement of the lower zone of the 

 oceanic segment would in turn tend to take 

 place, restoring in this manner a part of the 

 initial height. It is difficult to see, however, 

 how compression of the continental surface 

 could ever occur as a result from this spread- 

 ing except for local and minor adjustments, 

 since the flowing outward of the surface is the 

 operating cause. In so far as sediment from 

 the continents is deposited on the bottom of 

 the oceanic segments, however, and the sur- 

 faces of the two are brought toward a common 



level, the tendency of the top of the conti- 

 nental segments to spread outward will be 

 checked and a tendency for the lower part of 

 the oceanic segment to underthrust the land 

 will arise. But the greater part of terrigenous 

 sediments have been deposited within conti- 

 nental geosynclines, or upon the lower conti- 

 nental elements, or as submarine deltas build- 

 ing slightly outward the continental shelves. 

 The isostatic readjustment should, therefore, 

 be largely between the high and low conti- 

 nental elements and it is not clear that sedi- 

 mentation could account in any large measiu'e 

 for such a crowding together of the conti- 

 nental elements by pressure from the oceanic 

 segments as the structure of Asia seems to 

 suggest. Furthermore, WiUis has shown that 

 the present great relief of Asia is the result of 

 a very recent movement unconnected with the 

 Permo-Mesozoic folding and but partly origi- 

 nating in the mid-Tertiary period of erogenic 

 activity. It seems best, therefore, to consider 

 that vertical movements, bringing about iso- 

 static adjustment, are but indirectly connected 

 with the great compressive movements, follow- 

 ing after them to a greater extent than accom- 

 panying them; due to a seeking for an equi- 

 librium destroyed on the one hand by erosion 

 and sedimentation, on the other by changes of 

 specific gravity from within, induced partly by 

 the previous horizontal compression. Hori- 

 zontal compressive movements, on the other 

 hand, characterized by their large amount and 

 brief duration and separated by relatively long 

 periods of quiet, seem to have found as yet 

 no better explanation than that advanced by 

 Ohamberlin, as due to a progressive shrinkage 

 of the entire central portion of the earth, 

 resisted by the rigidity of an outer unshrink- 

 ing zone many hundred miles in thickness. 

 This gives the necessary mechanism for the 

 gradual storage of compressive stress and its 

 periodic discharge by yielding of the outer 

 zone, a yielding characterized by the mashing 

 of geosynclines and other lines of weakness 

 in the outer shell, and the underthrusting of 

 the continental surfaces by the lower and 

 structurally stronger oceanic segments. 



Joseph Baerell 



Yale "Univebsitt 



