GEOLOGY OF SARATOGA SPRINGS AND VICINITY II5 



and which is the cause of the Appalachian and Green Mountain- 

 Taconic fold systems but which also, as Uirich lucidly shows, causes 

 contraction of these tracts of the eastern continental coast by over- 

 thrusting, where deep-seated buttresses, as the Adirondacks are 

 opposed to the folds. 



One of the overthrust planes which have been observ^ed on the 

 Schuylerville sheet, namely, that separating- the Georgian rocks 

 from the underlying Ordovicic beds, can be traced southward along 

 the western edge of the Georgian to a locality near Schodack Land- 

 ing, about 15 miles south of Albany, where the Georgian belt termi- 

 nates near the Hudson. It is now an interesting fact that in the 

 direct southwestward continuation of the overthrust plane similar 

 overthrusts can be observed along which the Siluric and Devonic 

 rocks have been moved westward. Chadwick has described such a 

 fault from Saugerties,^ about 30 miles southwest from Schodack 

 Landing and between the latter place and the large overthrusts at 

 Rondout, described by van Ingen and Clark. If these overthrusts 

 all belong to the same orogenic movement, the latter is at least of 

 post-Devonic age and probably dates, like all the typical Appa- 

 lachian orogenic movements, from the close of the Paleozoic era, 

 or is still younger. 



THE NORTHUMBERLAND VOLCANIC PLUG 

 BY H. p. GUSHING 



One mile north of the village of Schuylerville, near the Hudson 

 river, is a knob of volcanic or effusive rock of quite exceptional 

 occurrence. Woodworth was the first geologist to recognize its 

 true nature and to describe it.- Since 1901, when his study was 

 made, a considerable part of the knob has been quarried away for 

 road metal and other purposes, exposing many features of the 

 occurrence which were not originally visible, and also furnishing 

 comparatively fresh rock for chemical study. The reader can 

 obtain some conception of the differences between the knob as seen 

 in 1901 and as it was ten years later by comparing plates 17 and 18. 



Woodworth proposed the name of Stark's knob for this hill. 

 The name is a convenient one, and will be here used in referring 

 to it. Since the appearance of his report, Schuylerville people 

 speak of it as " The volcano." 



1 G. H. Chadwick. Downward Overthust Fault at Saugcrties. N. Y. 

 N. Y. State Mus. Bui. 140. p. 157. 

 - \. Y. State Geol. 21st Rcpt. iqoi, p. r 17-24. 



