336 H. P. CUSHING — NORTHUMBERLAND VOLCANIC PLUG 



until studied by Euedemann and myself in 1910 while engaged in pre- 

 liminary study of the geology of the Schuylerville quadrangle. Between 

 the two dates a considerable part of the knob had been quarried away for 

 road metal and other purposes, exposing to view many structural features 

 which were entirely hidden before quarrying began, and also furnishing 

 comparatively fresh rock for microscopic and chemical study, in contrast 

 to the badly altered surface material which only was available in 1901. 



Woodworth proposed calling the hill "Starks Knob," and the name is 

 a convenient one. The occurrence well merits description, since it is 

 unusual and puzzling, quite unlike any other known occurrence of igne- 

 ous rocks in New York, and because it is of importance to determine 

 whether it has any significance in connection with the existence of the 

 mineral waters of the Saratoga region. 



Description 



Woodworth had to depend on weathered surface materials in making 

 his study; but since certain features of the rock were brought out by 

 weathering and were better exhibited at that time than is the case today, 

 we can not begin description of the occurrence in better fashion than by 

 citations from his paper : 



"Starks Knob igneous mass lies surrounded on the ground by Hudson River 

 slates. 3 These slates are highly inclined, cleaved and much broken rocks. . . . 

 So far as my own observations go, there are no dikes radiating from the main 

 mass into the sedimentary rocks, nor any noticeable sign of metamorphism in 

 them attributable to the heating action of the lavas of the plug. . . . This 

 lack of contact metamorphism and the failure of dikes are points of little value 

 in determining the origin of the igneous rock. It remains to determine by 

 other evidences whether it is intrusive or extrusive. 



"The igneous mass appears to be the superficial portion of a body which 

 extends downward into the slates and, from its general form and surround- 

 ings, strongly suggests a neck or plug rising up through the Hudson River 

 group at this point. The manner in which the slate body dips beneath the 

 igneous mass on the northeast appears to indicate that the plug does not ex 

 tend vertically downward through the slates, but follows guiding planes of 

 structure. 



"The exposed faces of the rock of the knob exhibit cross-sections of ball and 

 pear-shaped masses embedded in a base having a shaly structure. . . . 

 Where the shaly, fine-grained base has peeled away the surface resembles the 

 coarse, bulging surface of basalt streams, such as are seen in Hawaii. The 

 whole has the appearance of a mass of bombs or lava balls, inclosing scori- 

 aceous lava, or foreign inclusions embedded in a basaltic glass which is scaling 

 to pieces along lines of flowage. A more probable explanation of the structure 



3 These slates are chiefly of NormanskiU age, 



