1895.] 



881 



[Lyman. 



The Tardley Fault. 



By Benjamin Smith Lyman. 



{Read before the American Philosophical Society, September 6, 1S95.) 



1. Situation. 



2. Presumptive Importance. 



3. Color Break. 



4. Ocular Illusions. 



5. Dips of the Beds and Fault. 



6. Direction of Downthrow. 



7. Extent of Downthrow. 



8. Grouping of Rock Beds. 



9. Width of the Fault. 



10. Filling of the Fault. 



11. Fault not caused by Trap. 



12. Conclusions. 



1. There is a fault of striking appearance in the railroad cut of the 

 Bound Brook line of the Philadelphia and Reading Railroad nearly a 

 mile west of Yardley Station. It is the same as the fault that was 

 described by Prof. Henry Carvill Lewis before the Geological Section of 

 the Academy of Natural Sciences in March, 1880, as published, with a 

 figure, in the Academy's Proceedings for 1882, pp. 40 and 41. 



2. The fault seemed highly important, and he remarked at the outset, 

 "that it was not often that a section of a well-defined fault was exposed 

 for study. Frequently a fault starts a line of erosion which obliterates 

 ail traces of it, and llie actual junction of the faulted measures is either 

 occupied by a stream or is so covered by lalus that it can only be inferred 

 from adjoining outcrops." It is quite true that a fault, great or small, 

 must very rarely be exposed naturally ; though its place is not so apt to 

 be occupied by a stream as narrow river gorges or chasms often suggest ; 

 for they, perhaps, without exception, are merely the result of secular 

 erosion without any fault at the outset. Faults, too, are probably not in 

 general sudden disruptures leaving gaping abysses that may be filled by 

 streams and afterwards widened into important valleys ; but arise in 

 small movements and increase by slow degrees through many long ages. 

 Meanwhile the surface erosion accommodates itself gradually to the 

 changed circumstances according to the hardness or softness of the beds 

 that may be brought to light ; and the surface wash, except possibly in 

 rare cases on the face of a cliif, obscures the junction of the two un- 

 matched sides of the fracture. Artificial exposures of small faults are 

 numerous, as, for example, in the railroad cuts near Plicenixville and 

 Gwynedd ; but such exposures of great faults are necessarily rare 

 because great faults are comparatively rare ; and they must seldom be 

 encountered in railroad work, that keeps to the surface as much as 

 possible, though they may be found somewhat oftener in mines. The 

 presumption, then, in the case of a fault exposed by a railroad cutting 

 is that it is a small one, however important a look it may have. 



'S. The Yardley fault is so striking because a thick bed of bright 

 red shales on the western side abuts against light brownish gray shaly 

 and somewhat pebbly very soft saudrock on the east, with no lower 



