124 N^^V YORK STATE MUSEUM 



past one another except under the weight of a considerable thick- 

 ness of overlying rock. 



In addition to these two great zones there are a host of minor 

 shearing planes all through the knob, all showing slickensides on 

 their faces, with usually horizontal striae. The lava balls always 

 show exterior slickensides, and the intervening matter is every- 

 where greatly sheared. 



During 1910 great sheared surfaces became exposed at the base 

 of the excavations in the central part of the knob. The sheared 

 material had considerable thickness and consisted of a mass of 

 shaly, thin plates, with beautifully polished slickensides on all sur- 

 faces. The sheared zone had an easterly dip of about 40° and 

 seemed to pass with that inclination entirely through the knob, 

 from bottom to top. It was actually disclosed only for about one- 

 third of the whole height. Plate 19 is a photograph of a portion 

 of this sheared mass, showing the shaly material in considerable 

 thickness. At the time it did not occur to us that this was the 

 actual base of the trap, and we interpreted it as a shear zone 

 within the mass. At subsequent visits it was found covered largely 

 by quarry debris, so that it showed but poorly. We learned, how- 

 ever, that the material had been drilled into for a depth of 12 feet, 

 in search for additional trap underneath, but that none had been 

 found within that distance. This suggests that we have here the 

 actual base of the lava. The material does not exactly resemble 

 the ordinary shale, however, and there is some reason for the 

 belief that it represents a sheared mixture of shale and lava. Here 

 again the striae on the slickensides approach the horizontal. 



There is one more indication of dislocation in the knob ; its back 

 seems to be broken. This becomes quite evident to the observer 

 on the summit of the knob, looking down into the excavation. 

 The main shear zone in the northern half of the knob trends 

 N. 10° E., which is also the trend of the lava mass there. The 

 southern half, however, trends N. 45° W. and its main shear zone 

 does likewise. The knob appears as if it had been cracked in two 

 midway by a vertical rent, and the two halves twisted out of aline- 

 ment through an angle of 55°. 



All the evidence indicates that the lava of the knob has experi-' 

 enced compressive dislocation of the same type as have the over- 

 thrust shales in whose midst it lies ; of the same type and quite 

 comparable in amount. 



