Febbuart 18, 1910] 



SCIENCE 



245 



ering of the continental plateau, till the 

 present condition of approximate isostatie 

 balance were reached. 



The postulate of isostatie equilibrium 

 among masses of unlike densities in the 

 earth's crust has recently been strongly 

 supported by the work of Putnam^ and 

 Gilbert^ on gravity in the United States, 

 of Hayford* on the deflection of the plumb 

 line in the United States and of Hecker^ on 

 the attraction of gravity on the oceans. 



Hecker puts the general conclusion 

 thus: 



It follows that not only the superficial masses 

 of the continents must be compensated by a defect 

 of mass, a less density in the earth's crust under 

 the continents, but also that there is compensa- 

 tion beneath the deep seas through the greater 

 density of the ocean bottom. 



Inasmuch as it has been shown that Pratt's 

 [Dutton's] hypothesis of the isostatie relations of 

 masses holds not only for the continents, but also 

 for the three oceans (Atlantic, Pacific and In- 

 dian), we may regard it as a law which, apart 

 from certain disturbances, is a general one for the 

 earth's crust. 



This conclusion appears to place the 

 permanence of ocean basins outside the 

 category of debatable questions. 



A conclusion which follows closely from 

 that of the permanence of oceans, is the 

 constancy of the major oceanic drifts or 

 currents from an early date in each of the 

 great oceans. 



Movements of ocean waters result from 

 winds and differences of density of the 



^ Putnam, G. P., " Results of a Transcontinental 

 Series of Gravity Measurements," Phil. 8oc. Wash. 

 Bull, Vol. XIII., pp. 31-60, 1895. 



' Gilbert, G. K., " Notes on Gravity Determina- 

 tions by Mr. Putnam," ibid., pp. 61-76, 1895. 



* Hayford, J. P., " The Figure of the Earth and 

 Isostasy," U. S. Coast and Geodetic Survey, 1909. 



° Hecker, 0., " Die Sohweresbestimmung an der 

 Erdoberflache und ihre Bedeutung fiir die Ermit- 

 telung der Massenverteilung in der Erdkruste," 

 Zeitschr. der Gesell. fiir Erdkunde, Berlin, No. 6, 

 1909. 



waters. The trade-winds and their comple- 

 ments, the westerly winds of higher lati- 

 tudes, are due to causes which have existed 

 since the atmosphere and oceans formed ; to 

 rotation of the earth and to the distribution 

 of the sun's heat. These causes operating 

 through the winds on water bodies of 

 oceanic dimensions must have always pro- 

 duced an east-to-west equatorial current, 

 which being diverted by continents, de- 

 veloped great circulatory movements in the 

 several ocean basins, flowing clockwise in 

 the northern hemisphere and anti-clock- 

 wise in the southern. On the basis of the 

 arguments just presented, the ocean basins 

 are permanent, and hence the great super- 

 ficial oceanic currents which characterize 

 them must be regarded as equally ancient 

 in their main features. 



This conclusion regarding superficial 

 currents does not necessarily apply to the 

 deeper circulation, and there are reasons 

 for believing that the latter is now abnor- 

 mal. The deep-seated circulation is occa- 

 sioned by differences of density or head, 

 dependent upon temperature, salinity, pre- 

 cipitation and heaping of the waters by 

 wind. Chamberlin^ has suggested the 

 analysis of these factors and has brought 

 out the possibility of a change in the equi- 

 librium of the waters, which may have 

 resulted in warm highly saline currents 

 fioiving poleward from the equator, ie- 

 neath cool relatively less saline currents 

 flowing toivard the equator; the reverse of 

 the present condition. 



The density of polar waters is attributed 

 primarily to cold, and, as Chamberlin 

 points out, may be increased in those re- 

 gions where ice forms and where there are 

 no large rivers by the salts forced out of 



" Chamberlin, T. C, " On a Possible Reversal of 

 Deep-sea Circulation and its Influence on Geologic 

 Climates," Jour, of Geology, Chicago, Vol. 14, 

 1906, p. 363. 



