SECULAR MOVEMENTS 223 



to minor movements (p. 219). Other minor movements are doubt- 

 less due to local causes. 



Extent of the movements. Between the highest elevation of 

 the land and the lowest depth of 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 large tract exceeds five miles in depth, the range is eight 

 miles. This represents fairly the vertical range of differential 

 movement of large areas, though not areas of continental size. 

 The average height of the continents is about three miles above the 

 average bottom of the oceans, and this may be taken roughly as the 

 differential vertical movement of the segments of continental 

 dimensions. 



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 sea. Referred to this 

 datum plane, the continents have been squeezed up relatively about 

 two miles, and the basins have sunk about one mile. The total down- 

 ward movement, representing the total shrinkage due to increase of 

 , density, is quite unknown, but from theoretical considerations, it 

 would appear to be far greater than the differential movement. This 

 Vould mean that all segments have probably moved toward the 

 center, the basin segments about three miles more than the con- 

 tinental. 



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

 interest, for it has a theoretical bearing on the extent of the down- 

 ward movements. Every mile of descent of the crust represents 

 more than 6 miles (6.28 = 27r) shortening of the circumference. If 

 the vertical movements 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 lateral thrust and the 

 crumpling of the shell. How far does this go in explaining moun- 

 tain folds? The shortening represented by the folds of the Alps 

 has been estimated at 74 miles; 1 the shortening for the Appalachians 

 in Pennsylvania, not including the crystalline belt on the east, 

 at 1 6 miles; 2 that of the Laramide Range in British America at 25 

 miles. 3 



1 Heim, Mei h;inismus der Gebirgsbildung, p. 213. 



* Chamberlin, R. T., Jour. Geol., vol. 18, p. 255, 1910. 



J McConnel, Geol. Surv. of Canada, p. 33 D, 1886. 



