126 GEOLOGY. 



probably a rather high estimate, the average elevation of the folded tract before 

 degradation may be taken at 2.5 or 3 miles above the previous surface, corrections 

 for compression, extension, and possible downward thrust being neglected as 

 immaterial for present general purposes. The present width of the folded tract is 

 about 60 miles; its original width was therefore, by estimate, 100 miles. Now, a 

 shell that has been crumpled from a width of 100 miles to a width of 60 miles, and 

 in so doing has raised its surface 2.5 miles, on the average, must have been origi- 

 nally 3.75 miles thick, neglecting possible changes of density, etc. If it raised its 

 average surface 3 miles, it must have been 4.5 miles thick, qualifications aside. 

 A similar method may be applied to other mountain ranges for which approximate 

 estimates of crustal shortening and of the heights of the folded area are available, 

 and the results are of a similar order of magnitude. It appears therefore that the 

 phenomena of the folded tracts themselves indicate that only a very thin shell was 

 immediately involved in their formation. 



The conclusion that a shell of a very few miles only was involved in the crump- 

 lings of the folded mountains is so important as to invite more careful estimates 

 of the shortening of the crust, of the dimensions of the folds, and of the relative 

 elevations of the folded tracts than have thus far been made, involving new and 

 more critical measurements and perhaps new methods. Awaiting such im- 

 proved data, a shell of perhaps 3 to 5 miles thickness may be taken as representa- 

 tive. This is found to be in reasonable harmony with other considerations. 

 For example, a normal fold cannot well be less than twice the thickness of the 

 folded strata until these have been closely recurved upon themselves and thinned 

 by transverse pressure, and this only occurs when close appression takes place. A 

 selection of the available cross-sections of the Appalachian folds mapped to scale 

 in folios of the U. S. Geological Survey, measured liberally so as to include the 

 lowest dips of the flanks, gives an average width of about 8 miles, or measured 

 more closely so as to include the more distinctly folded portions only, about 6 

 miles. The halves of these fall within the range above given. The statistics 

 of other mountain ranges, so far as available, are of similar import. 



The shear-zone. — If we seek reasons for the limited thickness of the folded 

 shell, and for a shearing-zone so near the surface, they may perhaps be found in 

 the relations of the weight to the rigidity of the crust, or the ratios of weight to 

 crushing-strength and shearing-strength. Available data as to the strength of 

 rocks are very inadequate, and only tentative applications can be made. The 

 experimental tests have usually related to the crushing-strength, rarely to the 

 shearing-strength. They are also usually made on selected rock, well dried, and 

 are hence probably higher than the average rock in its native wet condition. 

 They are clearly higher than the native strength of rocks near the surface, where 

 much weakened by Assuring, leaching, and other superficial changes. At moderate 

 depths, however, these weaknesses disappear, and the rocks become practically 

 continuous and sound. At greater depths, compression probably increases the 

 strength, while increased heat tends to reduce it, but for the depths involved in 

 this discussion neither of these factors probably becomes vitally important. The 

 average crushing-strength of the granites that have been experimentally tested 



