Lyman.] ""-^ [Sept. 6, 



limit to the sandrock exposed and with no tliick bed of red shales in the 

 exposed section of nearly a hundred feet above. If, then, the down- 

 throw were to the west, it would have to be more than that hundred 

 feet, or the red slvales would be found in the eastern part of the cut. 



4. Certain ocular illusions give at first the impression that the d()wn- 

 Ihrow is really to the west. On the north side of the railroad, where, as 

 Prof. Lewis says, the fault is best observed, the slope of the cutting, 

 combined with the steep inclination of the fault and its direction, makes 

 the fault appear to rise eastward, or dip westward, as represented in his 

 figure ; and a westward dip would imply a westward downthrow. 

 Furthermore, as he mentions and represents in his figure, tlie light 

 brownish gray shaiy sandrock above the red shales has, or had some 

 years ago, the appearance of being turned up at tlie side of the fault, as 

 if forced into that position by a downthrow westward. But that appear- 

 ance is now less noticeable than formerly, and seems to arise from the 

 fact that lliere is a slight depression, probably less now than formerly, in 

 the side of the cutting just west of the fault. As the northwesterly dip 

 of the beds is nearly at right angles with the direction of the railroad, 

 tlie depression of itself brings the exposed edges of the layers to a lower 

 level than at the fault, and readily gives to an observer standing on the 

 railroad the impression that the layers just there dip away from tJie 

 fault more steeply than they really do. Indeed, the soutliwesterly 

 course of the railroad, a little more southerly than the strike of the 

 rock beds, tliereby rising slowly across the measures, makes his figure 

 give the impression that the rociis dip easterly, instead of north- 

 wester-ly. 



5. The rock beds here dip about twelve degrees, north about twenty- 

 seven degrees west (true meridian). The fault dips about seventy-seven 

 degrees, north about seventy-eight degrees east ; that is, with a strike of 

 about north twelve degrees west and an easterly dip, instead of the 

 northeasterly strike and westerly dip that the slope of tlie cutting is apt 

 to make one believe at first. The accompanying plate gives a geometri- 

 cal construction from those observations, carefully verified at visits 

 several years apart, and shows the true position of the fault and the 

 probable relation of the beds on both sides. 



6. As the fault dips easterly, the downthrow is beyond a doubt in the 

 same direction. For this is plainly not a reversed fault, longitudinal to 

 the strike, like almost every one of the anthracite region, caused by an 

 overthrust of sharply folded beds under strong horizontal compression ; 

 but is a normal fiult, transverse to the strike and occasioned apparently 

 by the unequal sinking of the beds into the underlying rock mass, plastic 

 under their enormous weiglit. There is no reason whatever to suppose 

 that the downthrow here is not in the direction of the dip of the fault 

 according to the almost invariable rule of normal faults. 



7. There is, then, no evidence at all that the extent of the downthrow 

 is more than a dozen feet, and the shallowness of the cutting together 



