:276 G. R. MAXSFIELD STRUCTURE OF ROCKY MOUNTAINS 



lachians of Pennsylvania and in the southern Rockies. All measnrements 

 were referred to sealevel, 6.000 feet or more below the present surface, 

 ;and for the purposes of the restoration it was assumed that the Cretaceous 

 beds present in the northeastern part of the area traversed by the struc- 

 ture section had been deposited over the entire area so traversed — an 

 assumption that seems reasonable. The beds employed for measurement 

 were relatively competent strata that come to the surface at many places. 



Considering first the effects of folding without allowance for the Ban- 

 nock overthrust or for minor faults, the west half of the section, corre- 

 :sponding with the upper part of the illustration, has been shortened 3.9 

 miles. Similarly the remainder of the section has been shortened 4.5 

 miles. Considered as a whole, an area 39 miles long has been compressed 

 by folding to 30.6 miles. The actual shortening by this process is 8.4 

 :miles, or 21.5 per cent of the original length. If the minimum horizontal 

 displacement produced by the Bannock overthrust (12 miles) be added 

 to this shortening, the total shortening by both folding and overthrusting, 

 •disregarding minor breaks, would be 20.4 miles, or 52.3 per cent of the 

 'Original lengi;h. 



The average restored height of the top of the Cretaceous beds above 

 ;sealevel is 6.7 miles for the western half and 5.7 miles for the remainder 

 of the section: the average height for the entire section is 6.1 miles. 



c h 

 X^sing the formula — = — , where c represents the amount of linear com- 

 e d 



pression, e the actual leng-th, h the height of the folded mass above the 



reference plane, and d the dej^th of the folded mass below the reference 



plane, we have the following equations : 



3.9 _ 6.7 . 4.5 _ 5^. 8.4 _ 6.1 



iTTO d~' l676~~rf~' 30.6 ^ 



d = 2^.1 d = 21.1 d = 22.2 



These results represent respectively the depths below sealevel of the 

 folded mass for the western half, the eastern half, and for the measured 

 area as a whole. 



If the minimum displacement produced by the Bannock overthrust be 

 added to the effect of folding, the value for d for the entire area measured 

 would be 9.1 miles instead of 22.2 miles. 



Van Hise^^ places the maximum depth of the zone of flowage for the 

 strongest rocks at 10,000 to 12,000 meters, equivalent to 6.2 to 7.4 miles. 



2^ C. R. Van Hise : A treatise on metamorphism. U. S. Geol. Survey Mon. 47, 1904, 

 j)p. 189-190. 



