Lewis] 



450 



[May 15, 



width. The writer is not aware that such an extensive exposure of a 

 fault-rock has been previously described. A few yards is usually given 

 as the greatest width to which a fault rock attains, although similar 

 instances will doubtless be found elsewhere. The great development of 

 this interesting formation along the Bucks County fault, leads to the con- 

 clusion that the process of faulting was a sudden event. The immense 

 pressure, which gave rise to the fault, would appear to have been relieved 

 by violent crushing and slipping, perhaps accompanied by earthquakes. 



The best exposure of this fault-rock is in the railroad cut immediately 

 east of Chalfont Station, on the Doylestown Branch of the Philadelphia 



and Reading Railroad — a locality 

 which will well repay a visit. The 

 accompanying sketch very rudely re- 

 presents the appearance of the fault- 

 rock at Cliis place. 



The principal cleavage planes are 

 vertical or with a steep dip, while 

 secondary cleavage planes cut in all 

 directions. These principal cleavage 

 planes are often curved and folded, as 

 is finely illustrated on the Neshaminy 

 creek between Bridge Point and 

 Chalfont. Some interesting photo- 

 graphs were obtained of these con- 

 torted cleavage planes. 

 The fault-rock crosses theNeshamlny creek several times, and the course 

 of that creek from Chalfont to Bridge Point appears to have been deter- 

 mined by the fault. At each place where the fault crosses the wind- 

 ing stream, an easily recognizable ledge of fault-rock appears, as for 

 example near the boundary of New Britain, Doylestown and Warrington 

 townships, or again near where the "State road" crosses the creek, or 

 again west of Bridge Point. 



The fault may be followed continuously across Bucks county in a curved 

 line, concave towards the north, from the county line of Montgomery 

 county to the Delaware. It passes through Chalfont and along the Ne- 

 shaminy to Bridge Point, as already described, being shown in the stream 

 at a point a mile above Godshalk'sdam ; also half a mile below Godshalk's 

 dam, also just above Castle Valley bridge ; also near Spruce hill ; again 

 near Bridge Point ; thence, less distinctly to the hill below Bennett's cor- 

 ner, where limestone first appears immediately north of the fault, in 

 the road-bed. 



At Bushington, half a mile farther East, the limestone is conformably 

 overlaid by Hudson River slates, on which the basal conglomerate of the 

 Trias reposes unconformably. Upper Triasslc red shales, dipping north- 

 east, occur south of the line of fault. North-east of Bushington, hills of 

 Potsdam sandstone rise from beneath the limestone to form "Buckingham 



