ESTIMATES AND CAUSES OF CRUSTAL SHORTENING 35 



tion, when the rocks had neared the surface by denudation, 

 other fractures formed which have not been cemented. 



Thus throughout regions in which injection or cementation 

 is extensive, there is complete evidence of important local 

 extension of the crust of the earth. Moreover, in the case of 

 the igneous material, it is certain that it acts as a wedge forcing 

 the material apart. It also is possible that the wedging effect 

 of cementation may not be unimportant. While it is probable 

 that upon the average the deformations which produce the frac- 

 tures taken advantage of by the entering material resulted in 

 shortening the crust of the earth, it is by no means certain that 

 in many cases at least the extension of cementation or injection 

 did not largely compensate for the shortening due to the 

 deformation. 



Shortening of Algonkia?i and Arche an rocks. — No one yet has 

 been bold enough to attempt a quantitative estimate of the 

 shortening represented by the older mountains, the stumps of 

 which only remain. But oftentimes it has been stated in a gen- 

 eral way that probably the pre-Cambrian folding, and consequent 

 shortening, is as great or greater than all subsequent folding. 1 

 While I am not able to disprove this conjecture, it seems to 

 me that the closeness of corrugation assumed as general for the 

 ancient rock is not justified by the facts. I shall separately 

 consider the Algonkian and Archean rocks because they are 

 so dissimilar. 



In many regions the Algonkian sediments are not closely 

 plicated. For instance, in the Lake Superior region, including 

 the Original Huronian district, the Keweenawan and Upper 

 Huronian sediments are very gently folded. The same statement 

 applies to other extensive areas of pre-Cambrian sediments in 

 Canada. The sedimentary rocks of the Adirondacks are more 

 severely folded, but still the folding is not close. The crystal- 



Bayley: Mon. U. S. Geol. Survey, No. XXVIII, 1896, Pis. VII, VIII, IX, XXIII, 

 and XXVI. 



*A criticism on the contractional hypothesis, by C. E. Dutton : Am. Journ. Sci., 

 Vol. VIII, 1874, p. 121. Origin of mountain ranges, by T. Mellard Reade : Lon- 

 don, 1886, pp. I33-I53- 



