GEOLOGY OF SARATOGA SPRINGS AXD VICINITY II9 



been removed since our previous visit; between this and 1912 yet 

 more was removed, until now comparatively little of the volcanic 

 rock remains above ground. In addition to our own study of the 

 occurrence we have had the advantage of visiting it with other 

 geologists, with Van Ingen and with Smyth in 1910, with Kemp 

 in 19 II and with Wood worth in 1912. 



The result of opening up the knob has been to bring to light 

 certain structural features that Woodworth could not possibly 

 have observed in 1901, and to furnish fresher material for micro- 

 scopic and chemical study than was available to him. Other 

 features showed better on the old, weathered surface of the rock 

 than in the fresher interior. 



Constitution of the knob. The knob consists of lava balls, large 

 and small, with intervening material. The balls range up to 2 feet 

 in diameter. At the time of our first visit many of them lay about, 

 being broken up by the quarrymen. In these we w^ere unable to 

 verify Woodworth's observation that they consisted of a dense 

 exterior and more porous nucleus, though these two layers ap- 

 peared plainly on the weathered surfaces which he saw. A prob- 

 able reason for this may be that in quarrying, this exterior crust 

 comes away w^ith the intervening matter, leaving the balls free 

 from it. The deforming agencies which have acted upon the 

 knob may well have had this result. The balls, as we saw them, 

 always had slickensided exteriors, and consisted throughout of 

 similar material. Judging from all the data, it seems probable 

 to us that the balls had originally a glassy crust and finely crystal- 

 line interior, and that shearing has separated the crust and nucleus 

 so that they do not come away together in quarrying. For the 

 most part the balls consist of dense, black rock, so finely crystal- 

 line that crystals are neither visible to the eye nor to the lens. 

 Exteriorly the balls are occasionally amygdaloidal, but there is 

 no great quantity having such texture. 



The intervening material has been badly sheared and crushed, 

 so that much of it has the appearance of slickensided, shaly matter. 

 But some of it is less damaged, especially in the vicinity of lime- 

 stone inclusions, which seem to have acted to prevent crushing; 

 and here the rock is always of glassy texture, a black pitchstone. 

 This is also sometimes amygdaloidal ; and this amygdaloidal glass 

 differs so greatly in appearance from the other amygdaloid, that 

 we took it in the field for variolite. This, however, it does not 

 seem to be. 



