DESCRIPTION 337 



is that this mass represents a volcanic throat or plug at some depth below 

 the actual crater, but not below the point to Which explosive products may 

 have fallen back into the volcano, there to become embedded in the still hot 

 lava. Certainly the gross structure of the rock recalls many lava sheets with 

 locally formed explosive products, and the same structure is to be observed 

 in the lava flows of the Newark series in the Connecticut Valley." 



Since Woodwortb/s study of the knob the rock has been utilized for 

 various purposes and much of it quarried away. We visited it in 1910, 

 in 1911, and in 1912. In 1910 active quarrying was in progress. In 

 1911 no work was being done, but much material had been removed since 

 the previous visit. Between this and the summer of 1912 yet more was 

 taken away, and now no great amount of the igneous rock remains above 

 the general ground surface. Other geologists have also accompanied us 

 to the spot — Van Ingen and Smyth in 1910, Kemp in 1911, and Wood- 

 worth in 1912. 



Constitution of the Knob 

 general character of the constituent material 



The knob consists of lava balls, large and small, with intervening 

 matter. The balls range up to 2 feet in diameter and perhaps average 1 

 foot. At the time of our first visit many of them lay about, being broken 

 up by the quarrymen. They consist of dense, dull, black lava, so finely 

 crystalline that crystals are neither visible to the eye nor to the hand lens. 



The intervening matter has been badly ground and crushed, so that- 

 most of it has the aspect of slickensided, shaly material. The less crushed 

 portions are always of glassy texture, consisting of black pitchstone. 

 Both balls and pitchstone are locally amygdaloid al, but the amygdules 

 are small and scattered. They are chiefly filled with calcite, as are also 

 the cracks, which everywhere ramify through the rock. 



INCLUSIONS 



Inclusions abound throughout the knob. Though it stands surrounded 

 by black shales, there are no inclusions of shale in it. All seen arc of 

 limestone, and limestone of but a single type, which effervesces immedi- 

 ately and briskly with acid and contains little or no magnesia. They 

 range in size up to pieces a foot or more in diameter. Those in the pitch- 

 stone are often much corroded by the lava. In the balls corrosive action 

 is much less prominent. 



Whence came this limestone, from whal formal ion of the region, and 

 why are there no inclusions of other rocks, more especially of shale? This 



