GEOLOGY OF SARATOGA SPRINGS AND VICINITY 



117 



this fault, but the figure is purposely drawn with some vagueness on the 

 extreme left of the igneous rock. 



To sum up the geologic relations of the Stark's knob igneous mass, it is 

 surrounded on all sides by the Hudson river slates. The principal mass is 

 relatively faulted down into these sedimentary rocks on the south and east. 

 To the eye there appears no distinct evidence of contact metamorphism ; yet 

 the mass appears to be the .superficial portion of a body which extends 

 downward into the slates and, from its general form and surroundings, 

 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, expressed in figure 9, appears to indicate that the 

 neck or plug does not extend vertically downward through the slates but 

 follows guiding planes of structure. It is conceivable that the igneous rock 

 once overlay the surface of the slates, has been tilted with them in one 

 of the orogenic movements of the region, and has subsequently been faulted 



Fig. 9 Cross section of Stark's knob, showing general relation 

 to the slates, and the gross ball structure of the mass 



and thus separated from other masses of igneous rock which are now 

 removed by erosion ; but this view is not borne out by the observed geologic 

 relations as now exposed. 



Structure of the Stark's knob rock. The rock of which Stark's knob 

 is composed is complex in structure. The exposed faces exhibit cross sections 

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

 The crust of these balls consists of a layer of a dense, dark colored basic 

 rock of the diabase type, surrounding a variable nucleus of ashy, rather 

 porous, pumiceous-looking lava in most cases, and more rarely an included 

 marginally absorbed fragment of white, semicrystalline limestone. 



The line of demarcation between these three elements in the rock structure 

 is usually very sharp and, where the shaly, fine grained base has peeled away 

 from the surface of the lava balls, the surface of the latter resembles the 

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

 The whole has the appearance of a mass of bombs or lava balls inclosing 

 scoriaceous lava, or foreign inclusions embedded in a basaltic glass which 

 has devitrified and is scaling to pieces along lines of flowage. A more 

 probable explanation of the structure is that this mass represents a volcanic 

 throat or plug at some depth below the actual vent or crater but not below the 

 point to which explosive products may have fallen back in the volcano there 

 to become embedded in 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 formation 



