GEOLOGY OF SARATOGA SPRINGS AND VICINITY 



12: 



If the great diagonal shear zone just described lies at the actual 

 base of the lava, as we suspect, then the lava forms a short, sheet- 

 like mass inclosed in the shales and dipping in conformity with 

 them, as shown in figure 14. 



This corresponds with Woodworth's original conception as may 

 be seen by reference to his figure (figure 9, page 117). The 

 present relief of the knob is due to erosion, the trap being more 

 resistant than the adjacent shales. The shale wedges in the lava 

 may be due to lava tongues running out into the shales from the 

 main mass, or they may be the result of dislocation. There is 

 shearing and faulting in plenty to account for all the observed 

 phenomena. 



The lava then seems to lie within the shales after the manner 

 of an intrusive sheet; yet it can not possibly be a sheet. In the 

 first place it is altogether too short. At the north it ends very 



Fig. 14 Diagram of inferred relationship of lava and shales in Stark's knob 



abruptly; a trench dug in the rock on the north slope of the knob 

 is entirely in shale. At the south it becomes involved with shale 

 wedges, and just south of the knob a poor exposure shows a thick- 

 ness of about I foot of trap interbedded with the shales never- 

 theless here also the termination is fairly abrupt. The entire length 

 along the strike from north to south is not over 200 yards. More- 

 over the nature of the lava itself, the ball structure and intervening 

 glass, decisively negative any notion that we may be dealing with 

 a sheet. The structure indicates surface lava, either a flow or a 

 volcanic neck, a deposit in the throat of a volcano. 



Microscopic characters of the lava. There are two chief varie- 

 ties of the rock of the knob, the finely crystalline, dull, black rock 

 of the balls, and the black glass of the intervening material. Both 

 are locally amygdaloidal, but we saw no such material in place, 



