I92I. No. II. THE STRANDFLAT AND ISOSTASY. 293 



pensated for b\ a relative excess of mass of the crust under the Ocean 

 floor, due to a higher density than that of the continental crust at corre- 

 sponding depths. 



In other words we may assume that the earth's crust has attained its 

 level of isostatic etiuilibrium, under the Ocean as well as under the con- 

 tinents. 



The theory of isostasy was placed upon a more solid j^hysical basis 

 of numerous reliable observations by F. R. Helmert [1908, 1909] and 

 especially by J. F. Hayford's report [1909] on "The Figure of the Earth 

 and Isostasy from ^Measurements in the United States", and bv later 

 publications by Hayford and Bowie. 



\\'h!le the theory has been accepted by most leading geophysicists, 

 the attitude of the geologist towards this theory has differed more. Some 

 geologists like Reid, in 191 1, and Becker, in 1908 and 191 5, accepted Hav- 

 ford's geodetic analysis and interpretation unreservedly, and even assumed 

 a nearly perfect isostasy to exist. 



Others, like Hobbs. in 1909, were more or less opposed to the idea of 

 isostasy, while some, like Bailey \\'iI1is 1T907, 191 1, 1920^, received the 

 theory with much reserve and criticism. 



Gilbert, who was an early advocate of a partial isostasy, tlunks that 

 the rigidity of the earth's crust can only permit of a regional compensation 

 distributed over wide areas, and Barrell arrived at very much the same 

 result. 



It wouM be out of })lace here to go more deeplv into the \"ivid dis- 

 cussion which has been going on, especially between American geologists 

 and geophysicists, about the theory of isostasy. The controversies show 

 that the observed apparent anomalies of gravity may be interpreted in 

 different ways, depending on the postulates as regards the vertical distri- 

 Inition of density in the lithosphère, on which the argumentation is based. 

 An instructive summarv of the outstanding points in this discussion has 

 been given by Barrell [i9i9al. 



It seems to me tliat the results of the synthetic discussion, though 

 interesting, must remain very uncertain as long as the speculations have 

 to be based upon assumptions regarding the conditions of the lithosphère 

 prevailing at great depths below the surface, \\hich our present knowledge 

 gives us no means of controlling. If these assumptions should prove 

 fallacious, as*they easily may, the whole structure of conclusions will be 

 more or less overthrown. 



The fact is that at present we do not know the conditions of the rock 

 at the high temperature and pressure prevailing at great deptlis below the 

 earth's crust: we know nothing about the degree of plasticity or viscosity 

 of the rock at these depths: aufl we flo not know even approximately the 

 vertical distribution of density in the lithosphère. 



