GEOLOGY OF SARATOGA SPRINGS AND VICINITY 



123 



fault must lie between it and the main mass of the knob. But 

 these other shale wedges within the trap mass suggest the query 

 whether the shale in Woodworth's section may not also be a wedge 

 with trap beneath. 



At the extreme north edge of the knob is another projecting 

 point, remaining unquarried. A portion of this point is shown at 

 the extreme right in plate 18, though most of it did not get into 

 the view. Figure 13 is a sketch of the relation here. 



It is not certain whether the shale here was originally a wedge 

 within the trap, or whether it formed the lower part of the shale 

 cover of the trap on this side, into which a thin sheet of the lava 

 was extruded, or whether it is due simply to dislocation. 



The shear zones. In the nearly vertical shale wedge shown at 

 the right in figure 12, the shaly material is greatly sheared and 



E 



Shale 



Trap 



Fig. 13 Sketch of relations of shale and lava on point at the north end of 

 the plug, showing a shale wedge with lava above and below 



slickensided, as are also the trap surfaces adjoining the shale. 

 The grooves and scratches on the slickensided surfaces are nearly 

 horizontal in attitude, showing that these two parts of the knob 

 have undergone lateral dislocation along a northwest-southeast line. 

 Within the northern half of the knob excavation has brought 

 to light another great shear zone, nearly vertical and bearing 

 north-south. There is no shale wedge here along which displace- 

 ment has taken place, but merely a shearing past each other of 

 the two adjacent parts of the knob, with production .of a highly 

 polished and striated contact surface. As in the previous instance, 

 the striae are nearly horizontal, in no observed instance departing 

 more than 20° from it. Here again there has been lateral dis- 

 placement under conditions of load ; in other words, it is incon- 

 ceivable that these surfaces of hard trap rock could have been so 

 thoroughly smoothed and polished when undergoing lateral slip 



