6 T. C. CHAMBERLIN 



II. The second embraces the complicated distortions and the 

 metamorphosed phases of the Proterozoic and Archean complexes. 

 Usually these are only partially accessible, and the profound 

 changes they have undergone present formidable difficulties in 

 addition. 



III. The third includes the deeper and more massive deforma- 

 tions of the earth body. These can be regarded as accessible only 

 in a logical sense by means of indirect evidences or remote intima- 

 tions, or by a priori considerations. 



I. In the more surficial of these three fields estimates of crustal 

 shortening have been m.ade from time to time in the past, but in 

 the main these have been confined to linear shortening; they have 

 not included the depths involved in the shortening. This is 

 necessary for computing their total quantitative values. Nor has 

 there usually been any determination of the under-configuration 

 of the distorted masses. This carries a very important part of the 

 specific significance which the diastrophism embodies. A notable 

 beginning has been made in adding these two neglected factors 

 and increasing at the same time the reliability of the estimate of 

 the linear factor.^ But the labor involved in these more adequate 

 determinations is so large that much time must pass before a 

 sufficient number of such determinations can be made available for 

 a total estimate of the shrinkage involved in even the limited field 

 to which the method is adapted. 



II. In the Proterozoic-Archean field there is little ground to 

 hope for any general application of these superior methods, partly 

 because of the large measure of concealment of the terranes and 

 partly because of the excessive intricacy of the structure and the 

 frequent changes in petrologic nature which render sharp identi- 

 fications of the borders of the several members of the terrane 

 throughout the whole folded tract impracticable. The difficulties 

 of this field are formidable in the extreme. No one, so far as I 

 know, has thus far had the temerity to offer an estimate of the 

 amount of shortening implied by the intricate crumpling of these 



' Rollin T. Chamberlin, "The Appalachian Folds of Central Pennsylvania," 

 Jour, of Geol., XVIII (1910), pp. 228-51; "The Building of the Colorado Rockies," 

 i6i"(i., XXVII (1919), pp. 145-64, 225-51. 



