POSITION OF THE LAVA 347 



structural and paleontologic, shows to have been overthrust into the dis- 

 trict from the east. The question naturally arises, May not the lava itself 

 have been overthrust into the region along with the inclosing shales ? We 

 can not definitely answer this question, but we must emphasize its possi- 

 bility. Were it the fact, certain of the structural features would receive a 

 simple explanation. As a result of the overthrust ing the weak shales have 

 become a jumble of small separate masses of varying age. The grit bands 

 have been rent asunder and now occur in widely separated fragments 

 through the shale, after the manner of inclusions. In such a mixture a 

 .fragment from some mass of igneous rock could easily be carried. The 

 shearing and dislocation of the lava, its abrupt lateral terminations, the 

 lack of dikes running out into the surrounding shales, and the shale 

 wedges in the lava would all be simply and readily accounted for. More 

 especially the difficulty of accounting for the inclusions in the lava, abun- 

 dance of one type of limestone fragments and entire lack of inclusions of 

 all other sorts, more especially those of shale, would be much diminished. 

 The manner of occurrence in the shales, also, as a short and compara- 

 tively thick mass which follows down the dip of the shales, would not be 

 so difficult of comprehension. The shales are so cleaved that they usually 

 come apart more readily on the cleavage planes than on the stratification. 

 It is this character that makes it so difficult to collect fossils from them. 

 It would seem that the explosive action occurring on the formation of a 

 volcanic vent through them would have opened them vertically along the 

 cleavage planes instead of following the inclined stratification slope. 



If the lava solidified where it now rests, the only possible explanation 

 of the occurrence is that we have here a volcanic throat or neck. The 

 nature of the rock and the small lateral extent of the mass render it cer- 

 tain that it is neither an intrusive sheet nor a flow. 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 com- 

 monly filled with agglomerate, tuff, or solid lava. We recall no account 

 of a neck filled with such material as that at Starks Knob. The usual 

 filling is much more distinctly fragmental in type and more diversified in 

 character. On the other hand, we know of no reason why material such 

 as that at the knob might not accumulate in a volcanic neck. 



Many features of the knob forcibly recall the characters of pillow lavas, 

 characters shown to be produced (in many instances at least) in surface 

 lavas when poured out so that they mingle with surface waters. Sphe- 

 roidal structure, production of glass, and explosive mixture with frag- 

 mental material from beneath, are prominent features of such lava-flows. 

 The great number of shrinkage cracks which everywhere cut the lava 

 balls is another feature possessed in common. 

 XXIV— Bull. Gbol. Soc. Am., Vol. 24, 19X2 



