358 GEOLOGY 



for resistance become less favorable, and hence the stress compels 

 continued yielding until it is fully eased. After this a period is 

 required for stress to accumulate, sufficient to produce another 

 general deformation. Meantime the minor stresses that may 

 remain, or may be produced by the great deformations, tend to 

 ease themselves and thus give rise to the minor movements (p. 354). 

 Other minor movements are doubtless due to local causes. 



The extent of the movements. Between the highest elevation of 

 the land and the lowest depth of the ocean, there is a vertical range 

 of nearly twelve miles. From the Tibetan plateau, where a con- 

 siderable area exceeds three miles in height, to the Tuscarora deep, 

 where a notable tract exceeds five miles in depth, the range is 

 eight miles. This may fairly represent the vertical range of differ- 

 ential movement of large areas. The average height of the con- 

 tinents is about three miles above the average depth of the oceans, 

 and this may be taken as the differential movement of the great 

 segments. 



If the protruding portions of the lithosphere were graded down 

 and the basins graded up to a common level, this level would lie 

 about 9,000 feet below the surface of the ocean. Referred to this 

 datum plane, the continents, with an area about half that of the 

 ocean depths, have been squeezed up relatively about two miles, 

 and the basins have sunk about one mile. The total downward 

 movement, representing the total increase of density of the earth, 

 is quite unknown, but from theoretical considerations, it would 

 appear to be far greater than the differential movement, which 

 means that all segments have probably moved toward the center, 

 the basin segments about three miles more than the continental. 



The extent of the lateral movements of the shell has a peculiar 

 interest, for it bears, theoretically, on the downward movements. 

 Every mile of descent of the crust represents more than 6 miles 

 (6.28 = 2?r) shortening of the circumference. If the vertical move- 

 ments were limited to the relative ones just named, the mile of 

 descent of the ocean basins would give but little more than 6 miles 

 excess of circumference for the lateral thrust and crumpling of the 

 shell. How far does this go in explaining mountain folds? The 

 shortening represented by the folds of the Alps has been estimated 



