3 2 



NEW YORK STATE MUSEUM 



It has occurred to the writer that the cause may have been a 

 lateral compression which caused most of the limestone beds to 

 become denser without being folded, while certain other layers 

 yielded by folding. This would seem to imply a greater rigidity 

 for the folded zones, but if anything the evidence points to less 

 rigidity. 



Regarding the Canadian occurrence above referred to Logan says : 

 " It would appear as if the layers after their deposit had been con- 

 torted by lateral pressure, the underlying stratum remaining un- 

 disturbed, and had then been worn smooth before the deposition cf 

 the next bed. Where the inverted arches of the flexures occur 

 some of the lower layers are occasionally wanting, as if the corru- 

 gated bed had been worn on the under as well as the upper side." 

 But it is difficult to see how such a lateral pressure could cause 

 certain layers to become highly folded and broken while the layers 

 immediately below them are undisturbed. Also the apparently 

 worn character of the upper and lower sides, mentioned by Logan 

 and which is likewise true of the Trenton Falls occurrence, is left 

 without explanation. 



It is thought that the folded structure at Trenton Falls was, in 

 reality, caused by a differential movement within the mass of the 

 Trenton limestone. That the whole body of the limestone has 

 been moved is clearly demonstrated by the existence of the thrust 

 fault at Prospect. It is easy to see how when the force of com- 

 pression was brought to bear in the region there would be a tend- 

 ency for the upper Trenton beds on the upthrow side to move 

 more easily and consequently faster than the lower Trenton beds. 

 For instance the portion A in figure 4 being separated from C by 

 an intermediate mass B of slightly less rigidity, would slide over 

 C and cause the portion B to become ruffled or folded. Occasion- 

 ally parts of B would become fractured or faulted. A similar 

 explanation would also apply to the lower folded zone. The 

 folded zones thus merely indicate horizons of weakness along 

 which the differential movement has taken place. 



As thus explained it is evident why the strike of the minor folds, 

 the strike of the fault, and the strike of the large low folds of the 

 region should all be parallel and why the contorted strata should 

 be so local in occurrence, because all these phenomena were pro- 

 duced by the same local pressure. The differential movement 

 would also readily account for the rubbed or worn character of the 

 upper and lower sides of the contorted zone. 



It is interesting to note that similar phenomena of contorted 

 between nonfolded strata have been observed in clay banks of 



