132 NEW YORK STATE MUSEUM 



If the lava solidified where it now rests, the only possible con- 

 ception of the occurrence which at all fits the facts of the case is 

 that it is a volcanic neck. But volcanic necks are usually nearly 

 vertical, instead of quite inclined, as in this case; they usually cut 

 across the bedding instead of following it; and they are usually 

 filled with agglomerate, tuff or solid lava. We do not recall any 

 account of a volcanic neck filled with material such as that at 

 Stark's knob. The usual filling is generally much more distinctly 

 fragmental in type and more diversified in character. On the 

 other hand, we know of no reason why such material might not 

 accumulate in a neck. 



Many features of the rock of the knob recall very forcibly 

 the characters of pillow lavas, characters produced (in many in- 

 stances at least) in surface lavas when poured out so that they 

 mingle with surface waters. Spheroidal masses of lava, production 

 of glass, and explosive mixture with fragmental material from 

 beneath, are the prominent characters of such lava flows. A great 

 number of shrinkage cracks through the lava pillows is another 

 feature possessed in common. 



At Stark's knob there is no sign of the basal admixture with 

 fragmental material, such as is found in many pillow lavas, unless 

 indeed the shear zone material at the base may represent it, crushed 

 beyond recognition, as an analysis perhaps suggests. Nor is the 

 lava as highly altered as most pillow lavas are; yet this difference 

 may be more apparent than real. Most such have been described 

 from natural exposures, and exteriorly the rock at the knob was 

 highly altered. The fresh material is due to quarrying and comes 

 from many feet below the original surface. 



Nor is the structure exactly comparable with that of pillow^ 

 lavas, the chief difference being in the greater quantity of inter- 

 vening material at the knob. We have seen that shearing has prob- 

 ably increased the apparent amount of this, the outer portions 

 of the balls cracking and shearing away, especially where com- 

 posed of glass. But when every allowance has been made for 

 this it remains doubtful whether the balls were ever as closely 

 packed and close fitting as in most pillow lavas. 



In weighing the evidence for or against the lava being in place, 

 with part of the evidence seeming to point one way and part the 

 other, we must confess our inability to come to any definite con- 

 clusion in the matter. The overthrusting seems a priori so un- 

 likely that our sympathies are entirely with the other view. But 



