Lyman.] . do* [Sept. 6, 



tainly have left at least some blocks of trap on the surface and no trap 

 blocks are known to occur anywhere within seven miles of the fault. 



11. He considers the fault to have "been caused by the pressure from 

 below of the molten trap," and in support says tliat near Taylorsville he 

 has "observed the dip of the red shales changed in the vicinity of a 

 trap dike," a whole quadrant in direction. A more thorough investiga- 

 tion, however, has shown tliat the trap there is undoubtedly an 

 overflow sheet conformable to the shales, and not a dike that has 

 changed the dip of tlie shales. He also says that "near Harleysville a 

 dike below the surface has metamorphosed the strata into black argil- 

 lite and reversed the dip to the south." It is now known that Harleys- 

 ville is near the axis of an important basin, or synclinal, somewhat 

 closely though not steeply folded, and is on the belt of Perkasie Shales, 

 that contain some dark and blackish beds, as well as many greenish ones, 

 through a great length of outcrop, often several miles from any trap, 

 and with no reason whatever to suppose the dark color to come from 

 dikes below the surface instead of from the character of the original con- 

 stituents of the shales, or to suppose the dips occasioned by the trap. 



12. It is clear, then, that the Yardley fault is simply a transverse or 

 normal fault, quite unconnected with any trap dike ; that the fault dips 

 easlwardiy ; that the downthrow is no doubt in the same easterly direc- 

 tion ; that notwithstanding the conspicuousness of the fault through the 

 contrasted colors of the rock beds, the amount of downthrow is probably 

 no more than about a dozen feet ; and that the extent of the fault along 

 its strike is consequently not very great. As the fault is nearly at right 

 angles with the strike of the rock beds, it would give, even if large, no 

 great support to the old idea that the apparent thickness of the New Red 

 might be due to a series of great faults parallel to the strike — an idea 

 that has made the least appearance of an important fault in the New Red 

 seem highly welcome to geologists. Such longitudinal faults are indeed 

 generally of greater extent than transverse ones; but, as they can arise 

 only from tremendous pressure on beds of great firmness, it is hard to 

 imagine their occurrence in a region of such gentle dips and weak shaly 

 beds as our New Red field. 



TJie Chalfont Fault Rock, So Culled. 

 By Benjamin Smith Lyman. 



(Read before the American Philosophical Society, September 6, 1S95.) 



1. Situation. 5. Southwesterly Dips in tlie Western 



2. Prof. Lewis's Description Cited. Photograph. 



3. The Two Photographs. 6. Southwesterly Dips Conlinned. 



4. Southwesterly Dips in tlie Eastern 7. Saddle and Basin. 



Photograph. 8. Conclusions. 



1. Just east of the railroad station at Chalfont, in Bucks county, Penn- 



