502 C. SCHUCHERT PALEOGEOGRAPHY OF 2SORTH AMERICA 



marked movements of this character, as is proved by the presence to this 

 day of these deposits over vast areas. Vertical uplifts of a minor order 

 probably occurred as isostatic compensation due to continental loading, 

 and possibly some of the emergences may be thus explained. The appear- 

 ance of the land Siouxia in the Siluric may also be due to a low vertical 

 uplift. 



It would seem, therefore, that the movements of the land do not develop 

 extensive continental seas, but rather the small seas occurring in times of 

 emergence. These are the introductory seas of a new cycle, the sea in- 

 vasions prophetic of the grand transgressions that are developed during 

 periods of quiescence, denudation, sea-loading or filling, and the storing 

 of earth stresses. The periodic discharge of these stresses is apparently 

 the cause of the rhythmic oceanic subsidence and a new arrangement of 

 the distribution of the seas over the continents. Tn other words, we agree 

 with Hayford 129 that the earth is "a failing structure.*' but it fails on a 

 grand scale, only periodically and to a very minor extent "'"every year, and 

 probably every day and every hour." In this connection it may be well to 

 give Hayford"? views on this subject. 



After an elaborate study of the geodetic evidence furnished by 50 T sta- 

 tions in the United States, which consisted of •'determinations of gravity 

 and of determinations of deflections of the vertical," Hayford 130 concludes 

 as follows : 



"The compensation of the excess of matter at the surface ( continents t by 

 defect of density below, and of surface defect of matter (oceans) by excess of 

 density below, may be called tbe isostatic compensation" (28). 



'"Let tbe deptb witbin which the isostatic compensation is complete be called 

 the deptb of compensation. At and below this depth the condition as to stress 

 of any element of mass is isostatic. that is. any element of mass is subject to 

 equal pressures from all directions as if it were a portion of a perfect fluid. 

 Above this deptb. on the other hand, each element of mass is subject in general 

 to different pressures in different directions, to stresses which tend to distort 

 it and to move it" (29). 



'"The evidence shows clearly and decisively that tbe assumption of complete 

 isostatic compensation within the depth of 71 miles is a comparatively close 

 approximation to the truth, that tbe assumption of extreme rigidity is far 

 from tbe truth— .bat the United States is not maintained in its position above 

 sea level by tbe rigidity of tbe earth, but is. in tbe main, buoyed up. floated. 

 upon underlying material of deficient density" (32). 



"The fact is established by this geodetic investigation that tbe isostatic ad- 

 justment brought about by gravity has reduced the stresses to less than one- 

 tenth of those which would exist if tbe continents and oceans were maintained 



:2; ' Hayford : Bull. Philosophical Society of Washington, vol. 15. 1907. p. 37, 

 130 Hayford : Proc. Washington Acad. Sci.. vol. S. 1906. pp. 25-40. 



