674 JOSEPH BARRELL 



but become operative during epochs of diastrophism. The com- 

 pressive movements, on the other hand, have pressed and welded 

 the positive elements together, the axial directions of folding 

 representing the compression of the negative zones lying between. 



The cause of the diastrophism Willis ascribes to differences in 

 specific gravity, restricted, according to Hayford's determination, 

 to the outer hundred miles of the earth's body; the vertical move- 

 ments being chiefly due to isostatic readjustment between the 

 several continental elements, the compressive movements being 

 due to the tendency of the heavier oceanic segments of the earth 

 to spread and underthrust the outer portions of the whole conti- 

 nental mass. This theory of the cause of lateral compression was 

 discussed by the present writer in a review of Willis' work,^ and 

 the objections stated against it there are in part the same as will 

 be elaborated farther on in the present article. 



Hayford took up the same subject in his address, delivered at 

 Minneapolis on December 29, 19 10, as retiring vice-president of 

 Section D (Mechanical Science and Engineering) of the American 

 Association for the Advancement of Science, the title of his paper 

 being ''The Relations of Isostasy to Geodesy, Geophysics, and 

 Geology.""^ This is a paper of broad scope intended to show how 

 vertical movements not in apparent accord with isostasy and also 

 movements of folding may be explained as secondary results of 

 isostatic adjustment and really in harmony with the hypothesis 

 of nearly continuous movement in a crust of low rigidity and of 

 almost complete isostasy. This part of his theory is essentially 

 the same as Button's but is elaborated in greater detail. 



Harmon Lewis called attention to the defects in this theory of 

 deformation,^ but Hayford made a rejoinder, positive and sweeping 

 in its style, to this and other lines of criticism by Lewis.'' 



The names of Button, Wilhs, and Hayford deservedly carry 

 much weight and must be accepted at their face value by geologists 



^Science, N.S., XXIX (1909), 257-60. 



2 Published in Science, N.S., XXXIII (191 1), 199-208. 



3 "The Theory of Isostasy," Jour. GeoL, XIX (1911), 620-23. 



'•John F. Hayford, "Isostasy, a Rejoinder to the Article by Harmon Lewis," 

 Jour. GeoL, XX (1912), 562-78. 



