Lynmn.l ^It) [Sept. 15, 



not wholly impossible, to detect, still more so to prove, the existence of 

 the fault, in spite of its great throw of over fourteen tliousand feet. 



The accompanying sketch map sliows, at least roughly, the prolonga- 

 tion of the fault nine miles norllieastward into New Jersey and of equi- 

 distant strike curves, a tliousand feet apart in level, on the bottom of dif- 

 ferent sets of Mesozoic shales north and south of tlie island-like mass of 

 Paleozoic limestone, of Pennsylvania formation No. II, from the positions 

 determined west of the Delaware by the recent survey. It has been pos- 

 sible to make, roughly, the prolongation of the lines without observing 

 any rock exposures in New Jersey ; because the topography shows the 

 geological structure very distinctly on the north of the fault, and witli 

 some clearness on the south. On the north, the long straight hills and 

 valleys show very plainly that the strike of the rocks continues almost 

 straight northeasterly in the same general course as on the Pennsylvania 

 side of the Delaware, and nearly parallel to the fault, but gradually bend- 

 ing more to the north. South of the fault the strike as shown by the topog- 

 raphy, though not very far from parallel to the fault, is evidently decidedly 

 less straight in the western edge of New Jersey, as it is also in Pennsyl- 

 vania ; but farther east becomes for a space straighter and more closely 

 parallel to the fault and to the northern strike. This structure of the 

 southern shales is confirmed by the topography outside the limits of the 

 map. 



The shales on the north of the limestone and fault belong to the same 

 set of beds, mostly soft shales as those near Norristown, and near 

 Yardleyville ; while those next south of the limestone and fault are of the 

 set of likewise red, mostly soft, shales that is seen near Pottstown, over- 

 lying the couple of thousand feet of generally harder and in good part 

 greenish shales of the Perkasie tunnel and its neighborhood, that them- 

 selves rest on the red shales of Lansdale. The trap masses given are 

 copied from the New Jersey State geological map, except that the limits 

 of solid trap in place have been conjeclurally restricted, according to our 

 experience in PennsjMvania, to only a portion of the whole space covered 

 by blocks of trap and its decomposed earth. 



It is very interesting to see how clearly the mere topograpliy shows the 

 geological structure, and so in conjunction with the ascertained relations 

 of the beds north and south of the limestone, makes the presence of the 

 great fault in New .Jersey to be known with certainty, in spite of its 

 otherwise thorough concealment through the similarity of the northern 

 and southern shales and of their strikes. The topography, indeed, gives 

 good indication of the geological structure far beyond the limits of the 

 little map, and \vould perhaps do so through all the New Jersey portion 

 of the older Mesozoic, in spite of the less pronounced variation in char- 

 acter of its beds as regards hardness than what we find among Paleozoic 

 rocks. Now that the older Mesozoic scries of beds lias been so fully 

 worked out in Eastern Pennsylw^ania, with several subdivisions of such 

 different color and texture as to be very noticeable in traversing country 



