1895.] dbO [Lyman. 



sylvania, a deep rock cutting begins and extends two or three hundred 

 yards eastward. It is the spot that the enthusiastic geologist and 

 aaiiable gentleoian, Prof. Henry Carvill Lewis, not long before he 

 was so lamentably cut off in his early prime, particularly mentioned in 

 his account of the immense fault that he supposed to affect the "great 

 trap dike across southeastern Pennsylvania, " described in his paper read 

 before our Society on May 13, 1885, and published in our Proceedings, 

 Vol. xxii, pp. 438-45G. 



2. He says (p. 449) : "The line of fiiult is marked by abnormal dips, 

 blackened and broken shales, ' slickensides,' and other evidences of vio- 

 lent disturbance, along its whole length. But its most characteristic 

 feature is the occurrence of a zone of typical fault rock. This very 

 interesting feature is composed of a mass of gray, shaly argillite, so 

 crushed and cracked in every direction, and so baked and changed in 

 character, that it has lost all traces of stratification. This peculiar rock, 

 evidently the result of movement at the time of faulting, is cut by innu- 

 merable cleavage planes, crossing one another at every conceivable 

 angle. The small and irregular angular blocks thus produced are very 

 generally covered by slickensides, the result of sliding motion. This 

 /aw Z^ rocA; marks the line of fault, when all other indications fail, and 

 has rendered it possible to fix the precise position of the fault from end 

 to end. It fills a zone one hundred feet or more in 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 conclusion 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 

 crusliing and slipping, perliaps ac- 

 companied by earthquakes. 



" The best exposure of this fault 

 rock is in the railroad cut immedi- 

 ately 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 represents the appear- 

 ance of the fault rock at this 

 place." 



3. The geological structure can be more accurately understood by the 

 help of the two new photographs here reproduced, showing a large and 

 the more difficultly interpreted part of the north side of the cut. They 

 were most obligingly taken, with his accustomed skill and excellent 



