1883.] '*'*'' [Lewis. 



fore that, at several points in Chester county, we find that iron ore has 

 been dug adjacent to the dj'ke. 



Thoughout the whole line, every feature of the dyke has heen remarka- 

 bly constant, and it is clearly to be regarded as a single dyke, interrupted 

 only by occasional .jogs. That these "jogs" are due to contemporaneous 

 faulting will be rendered probable upon a consideration of the great fault 

 in Bucks countj'-, to the description of which we may now proceed. 



It has long been surmised that a fault existed immediately south of the 

 long, narrow strip of lower Silurinn limestone, which suddenly nppears in 

 the midst of the Triassic area of Bucks county. This strip of limestone, 

 extending in a south-west direction from Limeport on the Delaware to a 

 point south eastward of Doylestown, was briefly referred to by Professor 

 Rogers, and has been mentioned by several geologists since, but never 

 satisfactorily explained. While some thought that it was due to a fault, 

 other geologists (Mr. C. E. Hall, etc.), supposed that it was an i?land in 

 the Triassic sea, adducing the triassic conglomerate to the north of the 

 limestone as evidence of this theory. 



The writer has devoted some time to the study of this interesting region, 

 and, with the able assistance of Mr. S. E. Paschall, of Doylestown, Mr. 

 John S. Ash, of Holicong, and others, has prepared a map of the area, and 

 has been able to trace for a distance of nearly twenty miles an important 

 fault, which has not only thrown up the whole thickness of the Trias, 

 exposing its basal conglomerate, but has brought to the surface its floor of 

 Pal£Eozoic rocks. 



The fault has a length of about nineteen and a half miles. Its extreme 

 western limit is near the point where the Neshaminy crosses the line be- 

 tween Bucks and Montgomery counties, about four miles west of Chal- 

 font. (Chalfont is called "Whitehall ville on the maps.) The line of fault 

 is not visible in Montgomery county, and its western terminus is in close 

 proximity to, and probably in actual contact with a horizon of blackened 

 shales, extending thence in a north easterl}-^ direction, and almost continu- 

 ously, across Bucks county to the great region of black shales (quarried for 

 curbing stones) about Point Pleasant, on the Delaware river. 



The line of f\iult is marked by abnormal dips, blackened and broken 

 shales, " slickensides, " and other evidences of violent disturbance, along 

 its whole length. But its most characteristic feature is the occurrence of a 

 zone of typical "fault rock." This very interesting feature is composed 

 of a mass of gray, shaly argillite, so cru.shed and cracked in every direc- 

 tion, and so baked and changed in character, that it has lost all traces of 

 stratification. This peculiar rock, evidently the result of movement at the 

 time of faulting, is cut by innumerable cleavage planes, crossing one 

 another at every conceivable angle. The small and irregular angular 

 blocks thus produced are verj^ generally covered by slickensides, the result 

 of sliding motion. Tins fault-rock marks the line of fault, when all other 

 indications fail, and has rendered it possible to fix the precise position of 

 the fault from end to end. It fills a zone one hundred feet or more in 



PROC. AMER. PHILOS. SOC. XXII 120. 3e. PRINTED SEPTEMBER 3, 1885. 



