ESTIMATES AND CAUSES OF CRUSTAL SHORTENING 37 



criterion upon which to make an accurate judgment, as bedding 

 is missing. As seen (p. 29), cleavage is no criterion upon which 

 to make estimates of shortening, and this is especially true of 

 monoclinal cleavage, and such monoclinal cleavage is found in 

 the Archean for great distances in various places, as for instance, 

 in the Blue Ridge and Piedmont plateau, in southwest Montana, 

 and in various areas in Canada. Also banding is no criterion, 

 for, as has been seen in this paper, and shown in another place, 1 

 the regular banding in the Archean rocks is in many cases prob- 

 ably due to cementation and injection. However, it is often 

 found in these ancient rocks that the secondary structures them- 

 selves, such as slatiness and schistosity, are folded into undula- 

 tions, but these are in most cases rather gentle. For instance, 

 the schistose structure of the Blue Ridge at Doe River is a 

 single anticline, and on the Nacoochee-Hiwassee section are 

 two anticlines separated by a syncline. The descriptions of 

 Emmons and King show the same simplicity of structure for 

 the Front Range of Colorado. The undulations of the schists 

 are so gentle that they took them to be the remains of sediments, 

 and gave an estimate of their thickness. 



Finally, wherever we find exceedingly irregular and intricate 

 structures in which no estimate can be made of the corrugations, 

 even of the secondary structures, we are sure to find intrusive 

 material intricately interposed, which may itself largely or 

 wholly compensate for the shortening which we see. 



I therefore conclude with the present state of knowledge, 

 that we are wholly unable to make any quantitative estimate 

 of the amount of crustal shortening involved in the deformation 

 of the Archean rocks. 



Longitudinal shortening of mountain systems. — In all past esti- 

 mates which have been made of shortening in mountain-making 

 only the transverse shortening has been considered, but in order 

 to obtain a true estimate of the effects of deformation, it is neces- 

 sary to consider the amount of longitudinal shortening. If it 



1 Principles of North American pre-Cambrian geology, by C. R. Van Hise : 16th 

 Ann. Rept., U. S. Geol. Survey, Pt. I, 1896, pp. 662-668, 684-688. 



