Lewis.] "itj J [May 15 



lying red shales dip 10° E. 10° N. ; and twenty feet yet farther south the 

 dip is 10° E. 15° N. Ripplemarks, fossil plants, and traces of coal in 

 thin seams were here observed. Other localities farther west showed 

 dips of 15° E. 10^ ]Sr., the shales often being dark and resonant when 

 struck. Near Bridge Point, on the south side of the fault, tliere is a dip of 

 15° E. 10° S. ; at Bridge Point some hard altered shales dip 30° E. 15° S. ; 

 and some dips to the east were noticed south of Chalfont. Mr. S. E. 

 Paschall reports that eastward dips continue in the strata south of the 

 fault nearly to its western termination. North of the line of fault the 

 normal north western dip of 10^ to IS^ is resumed. 



As already indicated, a blackening and induration of the triassic shales 

 accompanies the abnormal dips south of the fault. Another zone of 

 blackened shales crosses the county from Point Pleasant in a south-west 

 direction north of the North branch of Neshaminy creek. There is an 

 excellent exposure of this argillite near Point Pleasant, where it is largely 

 developed and is quarried for curbing stone. It rings when struck by a 

 hammer, and has a fine conchoidal fracture. The Indians used it for the 

 manufacture of their stone implements, and traces of an ancient Indian 

 workshop occur at Point Pleasant. 



That the induration of these shales is due to the proximity of a trap 

 dyke is a view often maintained, but the facts at command of the writer 

 appear to point to a different explanation. The zones of blackened shales 

 would seem to be coincident with lines of pressure and disturbance. Can 

 it be that pressure alone would suffice to produce the change? 



The fault and its accompanying phenomena having now been described, 

 we may briefly consider the Palaeozoic strata which it has elevated, before 

 returning to the trap dyke displaced by it. A section across Buckingham 

 valley, at its central point near Holicong, would show the following suc- 

 cession of strata, beginning at the fault: (1) Potsdam sandstone, form- 

 ing Buckingham mountain, being identical with the Potsdam sandstone of 

 the North Valley hill of Chester county, and dipping steeply N. W. ; (2) 

 Magnesian limestone or calciferous sandrock (Cambrian), dipping steeply 

 N. W., conformable with the Potsdam, and corresponding with the lime- 

 stone on the northern side of Chester valley ; (3) Trenton limestone, slaty 

 and fossiliterous, conformable with the last, and only differing from beds 

 of the same age on the south side of Chester valley in being non-meta- 

 morphic. No beds of marble occur as in Chester valley, but on the other 

 hand a number of characteristic Trenton fossils (trilobites, brachiopods 

 and corals) occur in it ; (4) The basal breccia of the Trias lies uncon- 

 formably on the last, and the junction is finely exposed on the farm of 

 H. Shepherd, at Holicong. It is formed of limestone fragments cemented 

 by red shale. It is common on the north side of the Triassic area in Penn- 

 sylvania (being identical with the so-called "Potomac marble "), but is 

 rarely observed on the southern edge. The writer has also observed it 

 south of Norristown on the Schuylkill Valley Railroad, where it exhibits a 

 small but beautiful fault ; (5) A narrow stratum of red shale, containing 



