342 



H. P. CUSHING NORTHUMBERLAND VOLCANIC PLUG 



lava and shales. 4 The present relief of the knob is due to erosion, the 

 lava being more resistant than the inclosing shales. The shale wedges 

 inclosed in the lava may be due to lava tongues running out into the 

 shales from the main mass, but more probably they are due to dislocation, 

 to a breaking up of the lava mass by faulting and shearing, with inter- 

 leaving of masses of shale. 



Figure 4. — Diagram illustrating Relations of Lava and enclosing Shales of Starks Knob 



The lava forms a sheetlike mass dipping to the east with the shales at an average 



angle of about 35° 



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

 intrusive sheet; but it is altogether too short for a sheet. It is cut off 

 in very abrupt fashion on the north; a trench cut in the rock on that 

 side, just beyond the knob, shows nothing but shale. On the south it is 

 cut off nearly as abruptly. Moreover, the nature of the lava itself, the 

 ball structure and the pitchstone, decisively negative any notion that we 

 are dealing with a sheet. It indicates either a surface flow or a volcanic 

 neck. 



Microscopic Character 



There are two chief varieties of the rock of the knob — the finely crys- 

 talline, dull, black rock of the lava balls, and the intervening pitchstone. 

 Both are locally amygdaloidal ; but all such material seen by us was found 

 loose on the dumps, and such as had frequent amygdules was not abun- 

 dant. The study of the exposures has led to the belief that the balls 

 may have originally possessed a glassy crust, and this was AVoodworth's 

 belief also. But if so, this has been sheared away by subsequent move- 

 ments, and at present all glassy material is between the balls. The bulk 

 of it has been greatly mashed and sheared. The balls also invariably 

 show slickensided exteriors. 



Thin-sections from the balls show a network of minute feldspar laths, 

 set in what was in some cases certainly, and in all cases probably, a 

 glass base. In the fine-grained rock from the. margins of the balls the 



4 21st Kept. New York State Geology, figure 2, p. r!9. 



