40 C. R. VAN HISE 



one recognizes that longitudinal shortening may be as important 

 as transverse shortening, this difficulty disappears. 



Finally, to know the real effect of the deformation in moun- 

 tain folding it is desirable to know not only the average close- 

 ness of the plications in two directions, but the depth to which 

 this average closeness has been observed. In short, in order 

 to obtain the most significant estimates of the effects of crustal 

 shortening, not one dimension of a folded mountain mass, but 

 three, length, breadth, and depth or thickness, should be taken 

 in account. So far as I know, two of these factors, length and 

 depth, have been wholly ignored in all estimates of shortening. 

 The reason for this doubtless is that the difficulties in the way 

 of the consideration of all these factors are insuperable, at least 

 at 'present. 



Shortening of removed formatio?is. — Another element of uncer- 

 tainty in giving estimates of crustal shortening is the unknown 

 shortening of the rocks which have been removed by denuda- 

 tion. If, as has been supposed, circumferential shortening is at 

 a maximum at the surface of the earth, the strata which have 

 been most deformed have been removed from time to time. It 

 would therefore follow, if we could estimate the amount of total 

 shortening to which the rocks of the crust have been subjected, 

 that this amount would fall short of the real shortening which the 

 surface of the earth had undergone. The erosive forces as they 

 cut off the mountain tops and distribute the material upon the 

 border of the sea, smooth out the earth's wrinkles of age. 



Co?icliLsion. — If the argument of the foregoing pages holds, 

 it is clear that we must begin at the beginning in making esti- 

 mates of the amount of crustal shortening involved in moun- 

 tain-making. The published estimates have ignored so many 

 factors which must be considered before an estimate can have 

 any quantitative value, that I am forced to the position that they 

 are little more than guesses. It is therefore concluded that the 

 amount of shortening of the crust of the earth, due to it's 

 deformation, is an entirely unknown quantity. By this I do 

 not mean to imply that the crustal shortening has not been 



