STRUCTURAL FEATURES 34 I 



than 20° from it. These two masses have also suffered lateral relative 

 displacement under conditions of load. 



In addition to these two great zones are many minor planes of shear- 

 ing throughout the knob, all with slickensided faces and with striations 

 approaching the horizontal. 



During 1910 quarrying work at the knob was in such stage as to expose 

 great sheared surfaces in the central part of the knob. The sheared ma- 

 terial had considerable thickness, 3 to 4 feet at least, and consisted of a 

 mass of shaly, thin curved plates, beautifully slickensided on all surfaces 

 for the entire exposed thickness. At the time it did not occur to us that 

 this was the base of the trap, and we interpreted it as a great shear zone 

 which cleaved the knob diagonally from base to summit, since it had an 

 easterly inclination of some 40° and apparently passed clear through the 

 knob. Plate 11 is a photograph of a portion of this sheared surface, 

 showing a considerable thickness of the material. At subsequent visits 

 these surfaces had become covered by quarry debris and showed but 

 poorly. We learned, however, that the material had been drilled into to a 

 depth of 12 feet in search of lava underneath, but that none had been 

 forthcoming. This leads to the belief that this is really underlying shale 

 and marks the actual base of the lava. The material does not exactly 

 resemble the ordinary shale, and there is some reason to believe that some 

 lava has been sheared in with the shale. In this zone, also, the striations 

 approach the horizontal. 



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

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

 stands on the summit and looks down into the excavation. The main 

 shear zone in the northern half of the knob trends north 10° east, and 

 this is also the trend of the lava mass there. The southern half of the 

 knob, however, trends north 45° west, and its shear zone likewise. The 

 knob appears as if cracked in two midway and the two halves dislocated 

 ih rough an angle of 55°. From the highest point of the knob, as shown 

 in the photograph, a line carried nearly vertically down the front, with 

 just a slight inclination to the right, will separate the knob into these 

 two halves. 



All the evidence indicates that the lava of the knob has experienced 

 compressive dislocation of the same type as have the overthrusl shales in 

 whose midst it lies; of the same type and quite comparable in amount. 



If the great diagonal shear /one just described lies at the actual base 

 of the lava, then the latter forms a sheetlike mass, inclosed in the shales, 

 and dipping in conformity with them, as sketched in figure I. This 

 corresponds with Woodworth's original conception of the relations of the 



