ESTIMATES AND CAUSES OF CRUSTAL SHORTENING 21 



case the material on the flanks is no longer held in position by 

 the tensile strength of the rocks on the crest, and under the stress 

 of gravity slides down the slope, and this results in corrugations. 

 The plications upon the flanks may thus be partly or fully com- 

 pensated by separation of the material along the crests. How- 

 ever, the plications may be inferred, in estimating crustal 

 shortening, to have extended to the part removed by erosion. 

 In this case the amount of crustal shortening due to folding 

 would be overestimated. In reality, the original length of the 

 strata was that of a gentle continuous curve of the order of 

 magnitude of the mountain mass. 



The question may be asked as to the reality of the existence 

 of the gliding effect above assumed as a result of the action of 

 gravity. In another place 1 I have fully discussed the forms of 

 the secondary folds which occur in composite anticlinoria and 

 synclinoria of the first order. It there appears that the second- 

 ary folds upon the flanks of the mountains so commonly have 

 attitudes which must have resulted from this gliding effect, that 

 the composite folds, the secondary folds of which show such 

 attitudes, have been called normal composite folds. This dis- 

 cussion cannot be here repeated, but if the argument given be 

 correct, the gliding effect due to gravity producing secondary 

 corrugations upon mountain flanks is a significant phenomenon, 

 and consequently the cause here assigned for overestimates of 

 the amount of crustal shortening is of importance. 



It is clear that in appealing to the force of gravity to pro 

 duce corrugations upon the slopes of the mountains, I am follow 

 ing Dana 2 and Reyer. 3 However, I do not follow the latter fully. 

 He makes the gliding the cause of the formation of mountains, 

 whereas, it is clearly an effect, following the mountain-making. 

 Material cannot glide down until it has been raised up. My 



1 Principles of North American pre-Cambrian geology, by C. R. Van Hise : 

 1 6th Ann. Rept. U. S. Geol. Surv., Pt. I, 1896, pp. 608-615. 



3 Geological results of the earth's contraction in consequence of cooling, by 

 James D. Dana : Am. Journ. Sci., Vol. Ill, 1847, p. 185. 



3 Theoretische Geologie, by E. Reyer : Stuttgart, 1888, p. 829. 



