L.ewls.1 44U [May 13, 



a line of loose weathered boulders, it is practically continuous along a line 

 seventy miles in length. 



It will be shown that in Bucks county, the dyke abuts against the 

 south side of a great fault of several thousand feet upthrow, and upwards 

 of twenty miles in length, and that, at a distance laterally of five miles, 

 another long dyke of identical composition and structure, as proved both 

 chemical!}^ and microscopically, abuts against the north side of the fault, 

 continuing thence to the Delaware river. If not the same dyke laterally 

 displaced, the two portions clearly belong to the same system, produced 

 by a single cause. 



The following description of the course of the dyke south-westward 

 from the Neshaminy creek, near Doylestown, is supplemented by an 

 accompanying map in which all the observed outcrops are plotted. 



The dyke forms a very prominent hill at the north end of what is 

 known as " Mundock ridge," at a point one mile south-east of the village 

 of Bridgepoint. The Neshaminy creek flows at its base, and the trap 

 forms a conspicuous hill, steep on the north side, from the top of which 

 there is a view of Doylestown and vicinity. Tbis is at the corner of War- 

 minster, Doylestown and Warwick townships. 



As nearly as can be ascertained the dyke is here about 100 feet wide. It 

 is a fine grained diabase (dolerite), finely crystalline, and, as shown by the 

 microscope, composed of lath-shaped crystals of plagioclase, enclosing 

 irregular grains of augite and grains and crystals of magnetite. It rings 

 like metal when struck, and covers the hillside with large fragments, gen- 

 erally well rounded by the process of concentric weathering, due to atmos- 

 pheric influences, and often much rusted on the outside. 



The dyke rises in the middle of tlie Triassic area, cutting through the 

 upper Triassic shales. It is clearly therefore of late or Post-Triassic age. 



The Triassic shales and sandstones, immediately adjacent to the dyke, 

 are not altered by it. Yet, in the vicinity, northward from the dyke and 

 about Bridgepoint, the shales are strongly baked and blackened, breaking 

 with conchoidal fracture and ringing when struck. They somewhat re- 

 semble, when so altered, a black limestone. 



These baked shales, however, cannot be proved to have any connection 

 with the dyke, as will be more definitely shown when we consider the 

 great fault against which the dyke here abuts. They seem to be, however, 

 connected with the fault, since all along it, for many miles, these baked 

 rocks occur, as if heat had been evolved during the faulting. On the 

 other han,d, no local metamorphic effects have as yet been observed any- 

 where along the sides of the dyke. The molten trap seems to have cooled 

 too rapidly to produce such effects. 



The dyke was followed from this point in a direction to west of south, 

 and was noticed as it crosses each road in the eastern corner of Warring- 

 ton township, passing a half mile east of Warrington P. O., forming the 

 " Mundock ridge." In Bucks and Montgomery counties the term " mun-' 

 dock " is universally given to the trap. The trap runs at first nearly due 



