240 ROLLIN T. CHAM BERLIN 



shell. The measured section will thus end with the western edge of 

 the Glenwood Springs sheet, i\ miles southwest of Glenwood Springs. 

 Since the country was corrugated into the lofty folds portrayed 

 by the reconstructed cross-section, the high-standing areas have 

 been subject to some relaxational movement. Portions of the 

 area have settled by the process of normal faulting. It is to be 

 noted that all the faults shown in the cross-section are located on 

 the limbs of anticlines or in the plateau-like region between State 

 Bridge and Glenwood Springs. The faults appear to shun the 

 synclinal troughs. This would imply a spreading of the anticlinal 

 arches accompanied by downslipping on the sloping flanks. That 

 more of these normal faults are located on the western limbs of 

 anticlines than on the eastern may have been a chance matter of 

 local conditions and so of little import, or may possibly point 

 toward a westward creep in the relaxational process. There are 

 several theoretical reasons why the latter might be looked for, 

 but the evidence is so meager as to merely raise the query. Since 

 the settling along these faults took place presumably after the folds 

 had attained their growth, the foregoing estimates of the height of 

 the folded tract have been made on the basis of the original folds. 



THE THICKNESS OF THE FOLDED SHELL 



Having estimated the crustal shortening and the vertical 

 bulging which have arisen from the folding, the depth of the folded 

 zone is to be calculated. According to our formula, the product 

 of the present horizontal length of the folded section times the 

 vertical uplift equals the product of the shortening times the depth 

 of the folded shell (Fig. 12). This is true except in so far as there 

 has been an actual change in volume due to the compacting of 

 materials under the comprehensive stresses. As discussed in a 

 previous paper, ^ the amount of shortening due to the mashing of 

 the strata and the compacting of materials during folding is rela- 

 tively small in comparison to that resulting from corrugation — 

 in all probability not more than 5 per cent. Furthermore, the 

 shortening due to the compacting of the rocks is more or less offset 

 by subsequent elongation of the strata arising from jointing, from 

 the opening of fissures, and the penetration of igneous intrusions. 



'^ Jour. Geol., XVIII (1910), 236-37. 



