r20 NEW YORK STATE MUSEUM 



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 2, appears to indicate that the neck or plug 

 does not extend vertically downward through the slates but fol- 

 lows 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 and thus separated from 

 other masses of igneous rock which are now removed by eros- 

 ion; but this view is not borne out by the observed geologic rela- 

 tions as now exposed. 



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

 Knob is composed is complex in structure. The petrography 

 which has been worked up by Professor Gushing is described 

 in the note appended to this article. What is here stated should 

 be viewed in the light of Professor Cushing's studies made 

 mainly after this paper was written. The exposed faces ex- 

 hibit cross sections of ball and pear shaped masses em- 

 bedded in a base having a shaly structure. The crust of these 

 balls consists of a layer of 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, semicrys- 

 talline limestone. 



The line of demarcation between the 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 on Hawaii. The 

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

 closing 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 



