THE YIELDING OF THE EARTH'S CRUST. 1 



By William Bowie, 

 Chief, Division of Geodesy, XJ. S. Coast and Geodetic Survey. 



The geological evidence is such as to justify anyone in conclud- 

 ing that the material of at least the outer portion of the earth has 

 moved from place to place during past geological periods. Material 

 that must have been laid down in the form of sediments in shallow 

 oceanic waters, as is evidenced by the presence of sea-shell fossils, 

 is now above sea level, in some cases to the extent of many thou- 

 sands of feet. Sedimentary rocks, which must have been laid down 

 in horizontal or nearly horizontal strata, are now much curved and 

 distorted. There could be cited many cases to show that strata have 

 not only moved in elevation but also horizontally. What has caused 

 these movements is one of the outstanding problems in geophysics 

 and geology. 



To the superficial view, erosion of a mountain region and conse- 

 quent sedimentation of a river delta present nothing but a tendency 

 to smooth out the earth by cutting down its elevations to fill up its 

 hollows. In short, the matter may appear as simple as the opera- 

 tion of a steam shovel and a line of carts grading a new city section. 

 Comparatively recent geodetic investigations referred to in what 

 follows lead us, however, to regard the matter as much more complex. 

 Even yet a great deal more evidence is required, but there seems to 

 be reason to think that the secondary consequences of erosion and 

 sedimentation are most far-reaching and significant, leading to pro- 

 found modification of the views of mountain building which for- 

 merly prevailed. 



ISOSTASY AND ISOSTATIC COMPENSATION. 



During the last half of the nineteenth century scientists dealing 

 with geodetic and geological problems advanced the idea that land 

 masses were higher in elevation than the bottoms of the oceans be- 

 cause of lighter material under the former than under the latter. 



1 Paper delivered at meeting of Philosophical Society of Washington, Mar. 11, 1922. 



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