ESTIMATES AND CAUSES OF CRUSTAL SHORTENING 39 



the Pyrenees and the Himalayas, it appears that the longitudinal 

 shortening is relatively more important than in the mountain 

 systems in which the ratios between length and breadth are 

 greater, as in the cases above mentioned. In the case of the 

 Pyrenees this is beautifully brought out by the memoirs of 

 Roussell, which show that cross folds are here important. 1 



In the introduction of the neglected element of longitudinal 

 shortening into the problem of crustal shortening, in mountain- 

 making, we have a factor which, contrary to those above con- 

 sidered, increases the total amount of crustal shortening. In 

 order to properly estimate the effect of the formation of a 

 mountain system upon the area of the surface of the earth, we 

 must know its length and breadth now, as compared with the 

 original length and breadth of the rocks making the mountains. 

 The amount of crustal shortening is then known in surface 

 area, the only proper unit in which comparison can be made, for 

 shortening along one line is of little importance unless it extends 

 over some finite distance transverse to that line. However, the 

 introduction of this element of longitudinal, shortening very 

 greatly complicates the quantitative estimation of the amount 

 of shortening of the earth, and such an estimation, in a direction 

 transverse to the mountains, as has been shown, is a sufficiently 

 difficult task if all the factors are taken into account which 

 should be considered. 



Although aside from the purpose of this paper, it may be 

 remarked in passing that one of the difficulties which have 

 appeared to confront geologists is not real. Geologists, assum- 

 ing that all shortening is in a direction tranverse to the length 

 of mountain systems, have been puzzled by the resultant con- 

 clusion that the shortening of the crust of the earth is so largely 

 concentrated in a direction transverse to the meridians. 2 When 



1 Etude stratigraphique des Pyrenees, by Joseph Roussell: Bull. Carte Geol 

 de la France, Tome VI, 1893-4, an d especially accompanying Pis. I to V. 



Etude stratigraphique des massifs Mantagneux du Canigou et de E'Albere, by 

 Joseph Roussell : Bull. Carte Geol. de la France, Tome VIII, 1896-7. 



2 A criticism upon the contractional hypothesis, by C. E. Dutton : Am. Journ. 

 Sci., Vol. VIII, 1874, pp. 122-123. 



