224 MOVEMENTS AND DEFORMATIONS 



These estimates cannot be taken as measurements, but they are 

 sufficiently close approximations to make it clear that the shortening 

 of the shell involved in mountain folding is large. These estimates 

 represent only that shortening of the circumference effected at cer- 

 tain times and places; the whole shortening of a circumference 

 involves the shortening implied by all the transverse folds on a 

 given great circle. Usually a great circle does not cross more than 

 one or two strongly folded tracts of the same age, from which it is 

 inferred that the shortening on each great circle at any one time 

 was concentrated largely in a few tracts running at large angles to 

 each other. If the folding of one of the main mountain ranges be 

 doubled, it may perhaps represent roughly the shortening for the 

 circle at right angles to it, for its own period of folding. If one is 

 disposed to minimize the amount of folding, the estimate of the 

 shortening may perhaps be put at 50 miles on a circumference, 

 for each of the great mountain-making periods; or, if disposed to 

 make the estimate large, the shortening may be put at 100 miles. 

 For the whole shortening since the beginning of the Paleozoic era, 

 perhaps twice these amounts might suffice. Assuming the cir- 

 cumferential shortening to have been 50 miles during a given great 

 mountain-folding period, the appropriate radial shortening is 8 

 miles. For the more generous estimate of 100 miles, it is 16 miles. 

 If these estimates are doubled for the whole of the Paleozoic and 

 later eras, the radial shortenings are 16 and 32 miles, respectively. 

 If these or similar figures are correct, it is clear that the surface of 

 the earth has sunk toward the center by an average amount greater 

 than that of the highest mountains above mean sphere level, since 

 the beginning of the Paleozoic era. The shortening for earlier eras 

 can hardly be estimated from present data. 



Causes of Secular Movements 



The volume of the earth is affected by two sets of forces, acting in opposition 

 to one another, (i) the concentrating forces , consisting of (a) gravity and (b) molec- 

 ular and sub-molecular attractions, and (2) the forces which resist concentration 

 consisting of (a) heat and (b) molecular and sub-molecular resistances. 



i. The centripetal forces. The best known of the concentrating forces is 

 gravity, which tends to bring all parts of the earth as near the center as possible, 

 the heavier beneath the lighter. The gravitative force of the earth causes a 

 pressure of about 3,000,000 atmospheres at its center, and lesser pressures at lesser 

 depths. Gravity acts all the time, and tends to bring about greater density 

 wherever molecular movement permits. 



In addition to gravity, there are attractive forces between molecules, atoms, 



