HYPOTHETICAL STAGES LEADING UP TO THE KNOWN ERAS. 127 



is about 25,000 pounds per square inch; that of the limestones about 16,000 

 pounds; and that of the sandstones about 6000 pounds; while that of shale, 

 which has scarcely been tested at all, because unfit for building purposes, is 

 appreciably less than that of sandstone. At the depths where the shearing-zone 

 appears to lie, crystalline rocks undoubtedly exist in higher proportions than 

 near the surface, and so the average ultimate strength undoubtedly rises toward 

 the higher figure given, as the depth increases. It is to be noted, however, that 

 among the crystalline rocks some have less strength than the granites tested. 



Now, at a depth of a mile the vertical pressure arising from the weight of the 

 overlying rock is about 6000 pounds per square inch, while the resistance of the 

 rock to crushing is probably as great or somewhat greater than indicated above. 

 At this depth, therefore, on the average, the weight of the rock will be insufficient 

 to deform the crust by crushing, and thus adapt it to the shrinkage of the interior. 

 The crust must meet the tendency of the interior to shrink away from it by a 

 tendency to develop a parting, and thus bring into play the principle of the dome 

 in its support, and thereby develop a tendency to lateral thrust. Now as prev- 

 iously shown (Vol. I, p. 581) , the strength of even the best rock is utterly incapable 

 of resisting the thrusts of the flat dome of the earth's crust when fully brought 

 into play. At 3 miles depth the weighting has increased to about 18,000 pounds 

 per square inch, at 4 miles to about 24,000 pounds, and at 5 miles to about 30,000 

 pounds. Within this range the weighting must approach, and perhaps pass the 

 crushing-strength of the average rock involved. Down to the point where the 

 weighting equals the crushing-strength, the tendency to arching with accompany- 

 ing lateral thrust, must take precedence over the tendency to crush. At points 

 appreciably deeper, where the weight is greater than the crushing-strength, 

 deformation by flowage is the normal result. The distinction here is analogous 

 to that between the zone of fracture and the zone of flow, as developed by Van 

 Hise and Hoskins, 1 but is not strictly identical with it, for here lateral thrust and 

 shearing-strength are essential factors. The zone of shear reaches higher than the 

 zone at which fracture becomes impossible under radial stress alone. The zone of 

 shear is perhaps identical with the lower part of the theoretical zone of fracture 

 and the upper part of the zone of flow. 



Folding in tracts of least resistance. — Now since the shrinkage of the interior 

 is a slow and essentially simultaneous process, the shearing-stress probably develops 

 an essentially simultaneous thrust over large areas, and as the shell offers much 

 resistance to local folding, a conjoint shear for large areas — in distinction from 

 numerous local independent shears — is developed, involving a common movement 

 of large portions of the shell toward some tract of least resistance, where relief 

 is found fo? the whole stress in a concentrated folding. Tracts of low resistance 

 are obviously found (1) at or near the borders of the continents, where the crust 

 is already flexed in its descent from the continental platforms to the ocean bot- 

 toms; (2) in tracts back from the borders of the continents where special flexures 

 have been developed by deep sedimentation and attendant down-bowing; and 



1 16th Ann. Rept. U. S. Geol. Surv., I, 1894-95, pp. 589-603, 845-859. 



