MOVEMENTS AND DEFORMATIONS OF THE EARTH'S BODY. 549 



of reference for relative segmental movements. Referred to this datum 

 plane, the continents, having an area about half as great as that of the 

 ocean depths, have been squeezed up relativel}^ about two miles, and 

 the basins have sunk about one mile from the ideal common plane. The 

 total do^^TLward movement, representing the total shrinkage of the earth, 

 is quite unkno^\TL from observation. It is probably very much greater 

 than the differential movement, as will appear from theoretical con- 

 siderations as we go on. 



The extent of the lateral movements has a peculiar interest, for it 

 bears theoretically on the shrinkage of the earth. Every mile of descent 

 of the crust represents 6 miles (6 . 28) shortening of the circumference. 

 If the vertical movements were limited to the relative ones just named, 

 the mile of basin descent would give but little more than 6 miles of 

 surplus circumference for lateral thrust and crumpling. How far does 

 this go in explaining the known facts? By measuring the folds of the 

 Alps, Heim has estimated the shortening represented by them to be 

 74 miles. ^ Claypole estimated the shortening for the Appalachians in 

 Pennsylvania, not including the crystalline belt on the east, at 46 miles ;^ 

 McConnel placed that of the Laramide range in British America at 25 

 miles, ^ and LeConte that of the Coast range in California at 9 to 12 miles. * 

 These estimates must be corrected for the thickening and thinning of 

 the beds in the process of folding, for the composite character of the 

 folds, and for the effects of shearing and faulting. These will in part 

 tend to increase and in part to decrease the estimates. The first effect 

 of horizontal thrust is to close up all crevices and compact the beds as 

 much as they will stand without bending. A part of the unusual thick- 

 ness which the beds of folded regions commonly show is i)robably due 

 to this edgewise compression. In experiments on artificial strata made 

 to illustrate foldings (Fig. 449a), the thickening of the layers is a very 

 appreciable part of the process, though probably natural beds do not 

 thicken in equal proportion. After the beds have been closely folded and 

 the thrust is athwart them, they are thinned and stretched on the 

 limbs of the fold. How far this and other causes of extension offset 

 initial compression is undetermined, and is differently estimated. It 



^ Mechanismus der Gebirgsbildung, p. 213. 



2 Am. Nat., Vol. XIX, p. 257, 1885. 



3 Geol. Surv. of Canada, p. 33 D, 1886. 

 * Elements of Geology, 5th ed., p. 266. 



