340 



H. P. GUSHING NORTHUMBERLAND VOLCANIC PLUG 



it is not certain whether the shale marks the actual base of the lava or 



whether it is just another shale wedge. 



In figure 2 a wedge of shale in the southern portion of the knob is 



sketched. It is probably squeezed up into the lava from the shale which 



underlies it on the west side. This 

 especial wedge is excessively mashed 

 and slickensided and the material may 

 be in part derived from the lava. 



At the north end of the knob a long 

 point remains unremoved because of 

 the included shale wedge sketched in 

 figure 3. This wedge Avas plainly near 

 the top of the lava, no great thickness 

 of which ever lay above it. 



The surfaces of the lava adjoining 

 the vertical wedge of shale shown in 



figure 2,— Nearly vertical wedge of figure 2 are highly smoothed, polished, 



Shale enclosed in Lava of StarJcs Knob ' -. . , . -, n „,, .... 



, , ^ . , and slickensided. The stnation on 



It does not show clearly in the photo- 

 graph, but lies toward the left end of the these surfaces is nearly horizontal, 

 exposed face, nearly over the projecting lowing that these two parts of the 

 point sketched in figure 1. ° 1 



knob have been pushed past one an- 

 other laterally under conditions of considerable load. This zone of shear- 

 ing bears northwest-southeast. Zones parallel to this run through the 

 knob, though lacking included shale, and in all cases with the highly 

 slickensided surfaces showing nearly horizontal striations. 



Figure 3. — Sketdt of Shale Wedge enclosed in Lara at the north End of Storks Knot) 



A long projecting point remains unquarried here, the inner portion of which shows at the 



extreme right in the photograph, the remainder being without the field of view 



In the northern half of the knob the excavations have brought to light 

 another great shear zone, also nearly vertical, but bearing north-south. 

 In this we have again the same thorough slickensiding, and again the 

 striations are nearly horizontal, in no observed instance departing more 



