FAULTS 199 



Faults. — Kiimmel* has described the great Hopewell and Flemington 

 faults, each of which has displaced the strata in the southwest many 

 thousand feet, and also the three faults that determine the greater part 

 of the northwestern boundar}^ of the Newark. Besides these it is known 

 that numerous small faults affect the strata, but these can seldom be 

 recognized except where the actual fissure or shear zone can be observed 

 in fresh exposures. When, however, a layer of distinct individuality, 

 like a trap sheet, is affected by a fault of any consequence, this fact is 

 often readily detected by the effects on the outcrop. 



Wherever the trap rocks are freshly exposed in cuts and quarries cer- 

 tain prominent north-south or northeast-southwest fissures are nearly 

 always found, and the crushed and slickensided materials between their 

 walls, varying from an inch or less to many feet in thiclcaess, give un- 

 mistakable evidence of displacement. In the majority of cases doubtless 

 the actual movement along these fissures has been small, and perliaps the 

 slight warping produced by the upheaval and tilting of these rocks would 

 be sufficient to account for many of them ; but from these slight displace- 

 ments to faults involving a throw of several hundred feet there is every 

 gradation. Many of the small irregularities of outcrop, particularly 

 those sudden angular offsets in the trap sheets, have long been ascribed 

 to faulting, and doubtless many others have had a similar origin. 



Almost without exception the known faults of the Newark in this 

 region have the downthrow on the east side of the fissure ; hence the great 

 Hopewell and Flemington faults of the southwest have caused a three- 

 fold repetition of a large part of the thickness of the Newark strata and 

 their included trap sheets, and at least half of the width of the Newark 

 belt along the Delaware river is due to this repetition. No such great 

 faults are known in the central and northeastern parts of the belt, but 

 displacements ranging from less than a foot to several hiindred feet are 

 of frequent occurrence in the trap sheets of the Palisades and the Watch- 

 ung mountains, and similar faults are occasionally observed in fresh ex- 

 posures of the sediments. It is altogether probable that the intervening 

 areas are as greatly affected as the trap outcrops, and hence that no incon- 

 siderable fraction of the width in the central and northeastern portions 

 of the area is also due to repetition on a small scale by numerous faults. 



The Trap Eocks 

 modes of occurrence 



The Newark area of New Jersey is dotted and striped almost every- 

 where with outcrops of the hard, dark colored rocks usually known as 



* Annual Report of the State Geologist of New Jersey for 1896, p. 78 ; 1897, p. 107. 



