AMOUNT OF HORIZONTAL DISPLACEMENT. 477 



ogenic activity. If these periods were separated by a long time-interval 

 considerable erosion must have taken place. Thus some indication is 

 afforded of the conditions which determined the character of the fault- 

 ing, and the conclusion reached by other lines of evidence is strength- 

 ened, that folding and minor thrust-faulting result from the compression 

 of a competent stratum under great load, while major thrusts result from 

 compression after erosion has removed much of the load from the rigid 

 beds and at the same time developed lines of weakness in them. 



Amount of horizontal Displacement. — As described in the paper above 

 cited, the thrust-plane of the Rome fault has itself been folded by subse- 

 quent compression. By reason of this folding certain portions of the 

 overthrust rocks which rest in the synclines have been preserved from 

 erosion and afford a minimum measure of horizontal displacement. If 

 such subsequent folding lias taken place in the case of the Coosa fault, 

 the resulting folds have either not yet been revealed by erosion or they 

 have been entirely removed. The latter is the more probable. Except- 

 ing the two small offsets, evidently produced by intersecting faults, the 

 course of the Coosa fault is remarkably direct, considering the low incli- 

 nation of the thrust-2:)lane ; but it is quite probable that if the surface of 

 this region were degraded only a few hundred feet further the outcrop of 

 the Rome fault-plane, now so remarkably sinuous, would be equally as 

 direct as that of the Coosa fault, and the latter may have had at some 

 earlier stage of erosion a course equally as sinuous as the former has at 

 the present time. 



The strongest evidence of great horizontal displacement on the Coosa 

 thrust-plane is found in the widely diverse character of contemporaneous 

 formations on its opposite sides. It will be recalled that the same faunas 

 are found in the Connasauga shale and in the formations underlying the 

 Coosa valley, but the rocks which contain these fossils differ widely in 

 ai)pearan('e and point to very difierent conditions of dejiosition. Two 

 explanations of these facts arc suggested : First, there may have been a 

 barrier of land between the two areas of deposition, so that the rocks of 

 the Coosa valley and^ those south of the Coosa fault were laid down in 

 separate, though contiguous, basins. Deriving their sediments from dif- 

 ferent sources, they would difi'er in lithologic character, while the faunas- 

 would be essentially the same. No trace of such a land barrier, how- 

 ever, has yet been found, and the rocks in question contain none of the 

 characteristic marks of littoral deposition. 



The second explanation of the above facts is that the rocks now occu- 

 pying adjacent areas at the surface were originally deposited in com- 

 paratively remote parts of the same sea, and that the observed lithologic 

 differences are due to the gradual change which is always found to occur 



