Niles.] 278 [April 5, 



elevation was from six to eight inches, and the arch measured from 

 sixteen to eighteen feet from base to base over the crest. It was 

 formed along the line of a vertical joint, which extends beyond the 

 limits of the quany. The contiguous edges of the bed were bent 

 upward, making an elevation which was a little more upon the north 

 side of the joint than upon the south, and a slight fault was in this 

 way produced. 



A study of the characteristics and conditions of the displacement 

 convinced me that it was of recent origin. I subsequently had my 

 conclusion confirmed by the testimony of a foreman in the quarry, 

 who had been an eye-witness of the progressive formation of the in- 

 teresting feature. The movement of the rock had been attended at 

 times, he said, by explosive sounds, and sometimes fragments of the 

 rock had been thrown into the air. 



The eastern end of this little axis of elevation was where it 

 reached the wall of rock, which forms one of the limits of the quarry. 

 The joint extends into this rock, as above stated, but the elevation 

 and faulting of the bed was scarcely perceptible at the base of the 

 artificial cliff. These facts indicate that the dislocation was not 

 caused by the weight of the adjacent overlying rock, but that the 

 removal of the upper layers in the quarry had permitted this lower 

 bed to yield to the pressure to which it was subjected. As the force 

 must have acted perpendicularly to the axis of the fold, so here also 

 we have evidence of an active lateral compression in a northerly and 

 southerly direction. 



There are other close joints running east and west in the floor of 

 the quarry, which are likewise lines of slight displacements in the 

 form of small faults, but the evidences of their recent origin are not 

 so conclusive. 



In one corner a channel has been excavated in the rock for the 

 drainage of the quarry. The cutting was made by drilling two lines 

 of nearly contiguous holes for the margins of the channel, and then 

 removing the intervening stone. Here, also, were clear evidences of 

 a lateral sliding, for the parts of the drill-holes remaining upon the 

 •edge of the upper layer were not vertically above the lower parts of 

 the same holes shown upon the edge of the under bed ; there was an 

 unconformability of position like that reported of the pot-holes. 

 Here again the facts evince the existence of a force acting in the 

 direction of the meridian. 



There can be no doubt that in quarries like those at Lemont, where 



