348 H. P. GUSHING NORTHUMBERLAND VOLCANIC PLUG 



. There is no sign of the basal admixture with f ragmental material, such 

 as is found in many pillow lavas, at Starks Knob. But the base is not 

 well exposed and, moreover, has been enormously sheared. An analysis 

 of this .basal, sheared material does suggest a possible mixture of lava and 

 shaly matter in its make-up, but this is not at all certain. The lava is 

 not as highly altered as most pillow lavas are, but this difference may be 

 more apparent than real. Most such lavas have been described from 

 natural exposures and exteriorly the lava at the knob was greatly altered. 

 The present fresh material was obtained from well within the knob, 

 available because of the quarrying. 



Nor is the structure precisely comparable to that of pillow lavas, the 

 chief difference being in the much greater quantity of intervening matter 

 separating the ellipsoids at the knob ; but there has been much shearing. 

 All the balls show slickensided exteriors and the greater part of the inter- 

 vening matter has been greatly sheared. It is highly probable that the 

 quantity of intervening material has been much increased by this means, 

 the outer, glassy portions of the balls being thereby sheared away. 



In attempting to weigh the evidence for or against the lava being in 

 place or being an overthrust mass, we must confess our inability to come 

 to any definite conclusion in the matter. The latter view seems a priori 

 so unlikely that our sympathies are entirely with the other. Yet the 

 similarity of the material to a fragment of a surface flow embedded in 

 the shales is so great and contrasts so strongly with the usual characters 

 of a volcanic neck that we can not relieve ourselves from the suspicion 

 that it is after all a fragment of a surface flow which was poured out on 

 a surface of Bald Mountain limestone, probably under water, and was 

 subsequently thrust westward to its present position. Under either view 

 there is no other mass with which to compare it. If it is in place it rep- 

 resents the only intrusion of its type or of its age that is known within 

 the State or anywhere in the vicinity. If it is an overthrust fragment, no 

 other fragments are known. They may be buried from sight elsewhere 

 in the thick mass of the overthrust shales or they may have been entirely 

 eroded away. 



The Inclusions 



The difficulty of accounting for the limestone inclusions has been al- 

 ready stated. The inclusions are many and all of the one kind of rock, 

 rock which seems referable to the Bald Mountain limestone perhaps, but 

 certainly to no other of the formations of the region. Just what the 

 thickness of the overthrust shales may be at the knob we have no means 

 of knowing, but they extend 5 miles farther to the westward before being 

 replaced at the surface by the undisturbed Canajoharie shales of the in- 





