STRATIGRAPHIC CLASSIFICATION DIASTROPHIC CRITERIA 433 



placements of adjoining segments of the earth's crust. As these phe- 

 nomena have, as a rule, no direct bearing on stratigraphic classification, 

 an extended discussion of the subject would be out of place here. How- 

 ever, there is one rather common type of normal faulting that is often 

 found in areas of sedimentary overlaps and which, in the matter of 

 location of the fault-plane, is thought to be connected with sediment 

 loading, namely, faults of the type referred to are very commonly found 

 short distances off the shores of old embayments. Good examples are 

 seen along the overlapping edges of the Paleozoic formations in central 

 Texas, on the flanks of Ozarkia, and on the west side of the valley of Lake 

 Champlain. They occur also, though less typically developed, on the 

 flanks of the Cincinnati, Nashville, and Wisconsin domes. The displace- 

 ment in none of these cases exceeds 300 or 400 feet and as a rule is less 

 than 200 feet. Where the throw is small great care is sometimes required 



Figure 15, — Diagram illustrating Hypothesis of Origin and Location of normal Faults 

 thought to have teen caused hy Relief from Stresses which developed through loading of 

 negative Areas. 



to distinguish the fault from the edge of the stratigraphic overlap. The 

 upthrow in these faults is always on the land side of the embayments. 

 This fact has suggested the following hypothesis concerning their origin. 

 The additional load of accumulating sediments in the synclinal embay- 

 ments, hence in areas already relatively dense and consequently of negative 

 tendency, disturbed the isostatic equilibrium in the region, and caused 

 spreading and flow of deep-seated matter toward the anticlinal areas 

 which are of less density and tend usually to move in a positive direction. 

 Under the circumstances fracture of the thinning crust, resulting in 

 normal faulting, is likely to occur near the margin of the protecting sheets 

 of overlapping sediments. Strictly, the movements as conceived do not 

 contemplate tension, which is commonly assumed to be a prerequisite in 

 the process of normal faulting. There is some expansion of the surface, 

 but the excess is accounted for in the general doming of the areas ad- 

 jacent to the fault. The upthrow on the landward side of the fault being 



