1885.1 ^'^^ [Lewis. 



are more or less conformable to the outcrop of the Jericho Hill dyke, 

 changing in dip as the outcrop of the dyke varies in direction. As exam- 

 ples of Triassic shales dipping away from the concave side of Jericho hill, 

 the following dips, observed by the writer in Upper Makefield township, 

 may be recorded. Immediately north of the western end of the dyke, the 

 red shales dip 15° S. 40° W. and, one hundred feet away, 32° S. 35° W., 

 the latter on the farm of A. Smith. On the border ot Upper Mukefleld 

 and Solebury townships, near Pidcocks creek, somewhat over a mile west 

 of the eastern horn of the dyke, dips of 22° N. 45° W., and 25° N. 30° W., 

 were observed. 



It appears therefore that the crescentic shape of Jericho hill is coincident 

 with a flat anticlinal fold :n the Trias, the axis of the fold dipping west- 

 ward. But if this is the case, the conclusion is probable that the trap is 

 an overflow sheet, overlaid and underlaid by shales, which, with the inter- 

 calated trap, have been subsequently folded. The crescent form of the 

 outcrop of the trap would be the natural result of the erosion of a west- 

 ward dipping flat anticlinal fold.* These crescentic dykes would thus be 

 of inter-Triassic age, and therefore older than the long dyke across the 

 State. 



Another short narrow dyke, which cuts through the limestone in D. 

 Ely's quarry in Solebury township, is noted in the accompanying map, 

 but it has no connection with and is of different composition from the long 

 dyke. 



The two dykes with which this paper especially deals, viz : that extend- 

 ing from the Maryland border to the Bucks County fault, and that extend- 

 ing from Chalfont to the fault again, constitute, whether we consider 

 them as one dyke displaced, or as two dykes separated by a fault of con- 

 temporaneous age, a feature of considerable geological interest. They 

 clearly represent, as has been seen, a great crack, due to separation of the 

 strata, being precisely the reverse of the action which produced the fault, 

 where evidences of pressure abound. No facts were observed which dem- 

 onstrate that the crack occupied by the trap was a line of fault. On the 

 other hand, we have seen that where an unmistakable fault cuts oft" the 

 dyke, such fault has not been entered by the trap. The fault was fllled 

 by a compact mass of its own rubbish — the "fault-rock." 



An examination of the relations between the Bucks County fault and the 

 trap dyke divided by it, renders it probable that they are of contempora- 

 neous Jurassic age. A mere inspection of the accompanying map leads to 

 the conviction that the bow-shaped dyke north of Buckiugliam valley is 

 due to the filling by trap of a crack caused by the uptilting of the Palaeo- 

 zoic and Triassic strata at the time of the great fault. It is just as if a 

 cake of ice on a pond had been forced up at one side, at the same time 



* The I'elatious between crescentic Triassic trap dylses and the adjoin ing strata 

 and the distinctions between intrusive and overflow dylces liave recently been 

 discussed by Mr. W. M. Davis in a paper on Triassic Trap Dykes and Sand- 

 stones (Bull. Mus. Comp. Zool. ix). 



