1895.J ^od [Lymau. 



with the talus at its sides does not make it certain that the downthrow is 

 more than half a dozen feet. The light brown soft shaly sandrock over- 

 lying the dark red shales is likely to be the lower part of llie similar 

 rock bed that they abut against at the fault; but it is not known how 

 near the top of the dark red shales comes to the surface on the east of the 

 fault, nor whether it may not even exist there above the level of the 

 railroad bed, but concealed by the talus at the bottom of the cutting. 

 As such faults are apt to be small, and as none of more than a few yards 

 have been found among the numerous exposures of like faults in other 

 railroad cuttings of Bucks and Montgomery counties, it is highly proba- 

 ble that the downthrow here is not more than about a dozen feet. 



8 The rocks, therefore, west of the fault are almost wholly lower 

 beds than those east of it ; and it is not strange that they should be of 

 different character. Those on the east are in great part light gray sand- 

 rock, weathering light brownish gray, with mustard-seed quartz grains, 

 and much decomposed white feldspar and black grains, apparently 

 decomposing hornblende and mica, and are in some parts very pebbly 

 with pebbles of quartz and black metamorphic rock. The materials seem 

 to be those of the gneiss only a mile distant to the south. The rocks 

 west of the fault are in great part red and shaly, but in part light gray 

 and light brown sandy shales. There is not liere any marked division 

 " between the lower white conglomerate and the overlying red shale," 

 as suggested by Prof. Lewis ; for the gray somewhat pebbly sandrock on 

 the east of the fault belongs to much higher beds than the more decided 

 conglomerates a few hundred yards to the south. There are many 

 alternations of red and gray hereabouts and the change at the fault is 

 but one of them. 



9. The fault on the north side of the cut is perhaps two feet wide 

 towards the top, but about half way down widens with irregular outline 

 to perhaps four feet, as exposed in 1887, or to 5^ feet, as stated by Prof. 

 Lewis in 1880. At present the loose earth, or talus, partly conceals the 

 middle and wider portion of the fault. 



10. The width of the fault is filled with loose, crumbling, mostly inco- 

 herent materials that are in the main of a very dark or blackish brown 

 color at the widest part. Prof. Lewis took the material to be decomposed 

 trap, and the fault to have been filled by a trap dike. On close exami- 

 nation, however, the material is seen to contain much quartz in small 

 colorless grains of irregular and rounded shapes, dark mica scales, much 

 decomposing, very soft white, and in part slightly yellowish feldspar, at 

 least partly plagioclase and perhaps wholly so ; and the dark brown por- 

 tion seems to be decomposing hornblende. The constituent particles, 

 then, are all such as are found in the neighboring gneiss and are mostly 

 to be distinguished in the sandstone just east of the fault ; and have no 

 doubt been washed into the crevice occasioned by the fault. The 

 occurrence of so many quartz grains is of itself proof that the material 

 could not be decomposed trap. Besides, a trap dike would almost cer- 



PROC, AMKR. PHILOS. SOC. XXXIV. 149. 2 W. PRINTED DEC. 6, 1895. 



