MOVEMENTS AND DEFORMATIONS OF THE EARTH'S BODY. 551 



right angles to a given folded tract. In so doing, it will be seen that 

 the belt does not usually 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 was largely concentrated in a few tracts running at 

 large angles to each other, to accommodate the shrinkage of the globe 

 in all directions. If the folding in a main range crossing any great 

 circle is doubled, it will probably represent roughly the shortening for 

 that entire circle for that age. If one is disposed to minimize the amount 

 of folding, the estimate may perhaps be put roundly at 50 miles, on an 

 entire circumference, for each of the great mountain-making periods. 

 If, on the other hand, one is disposed to give the estimates a generous 

 figure so as to put explanations to the severest test, he may perhaps 

 fairly place the shortening at 100 miles, or even more. For the 

 whole shortening since Cambrian times, perhaps twice these amounts 

 might suffice, for while there have been several mountain-making periods, 

 only three are perhaps entitled to be put in the first order, that at the 

 close of the Paleozoic, that at the close of the Mesozoic, and that in the 

 late Tertiary. The shortening in the Proterozoic period was consider- 

 able, but is imperfectly known. The Archean rocks suffered great 

 compression in their own times, and probably shared in that of all later 

 periods, and if their shortening could be estimated closely, it might be 

 taken as covering the whole. Assuming the circumferential shorten- 

 ing to have been 50 miles during a given great mountain-folding period, 

 the appropriate radial shrinkage is 8 miles. For the more generous 

 estimate of 100 miles, it is 16 miles. If these estimates be doubled for 

 the whole of the Paleozoic and later eras, the radial shortening becomes 

 16 and 32 miles, respectively. 



THE CAUSES OF MOVEMENT. 



General Considerations. 



The volume of the earth is at all times dependent on two sets of 

 antagonistic forces, (1) the attractive or centripetal, consisting of 

 gravity and the molecular and sub-molecular attractions, and (2) the 

 resistant forces — which are not necessarily centrifugal — consisting of 

 heat and the resistant molecular and sub-molecular forces. 



