Mesozoic Igneous rook of JVew Jersey. 823 



■which a little farther on emerge small upright columns more or 

 less inclined. The tops of the lower massive columns bend 

 away from the central mass just as those to the south, the junc- 

 tion of the two systems having been finely exposed quite 

 recently. Where the small vertical columns connect with the 

 lower ones, the rock is seen to be continuous from one into the 

 other. The system of cracks alone changes and most of the 

 upper cracks come to an end without causing a line of demarca- 

 tion between the upper and lower columns. 



A second and in some respects more interesting exhibition of 

 curving and radiating columns is to be seen about a mile and a 

 half from that just described, in the Undercliff quarry in Llew- 

 ellyn Park near the north gate. The same layer of rock is 

 here exposed in a section from 80 to 100 feet high by a cliff 

 some 700 or 800 feet long. The lower half is divided by two 

 quite regular sets of cracks into broad rectangular masses. 

 Less uniform partings in other directions are occasionally no- 

 ticed, but no distinct columns have been developed. The 

 upper half, on the contrary, is divided into small columns ten or 

 twelve inches thick, which radiate downward from foci at the 

 surface 50 to 100 feet apart. As many as seven or eight cen- 

 ters can be distinctly made out along the face of the exposure. 

 The general appearance at a distance is that of a layer of col- 

 umnar rock resting upon a massive one, with a well-marked 

 line of contact between them, but closer inspection shows that 

 no such line of separation exists, the columns coming down un- 

 evenly upon the lower mass at all angles from vertical to hori- 

 zontal, and the rock of the upper and lower portions passing 

 uninterruptedly from one to the other. The apparent differ- 

 ence arises solely from the systems of cracks which divide each. 

 The best exposure in this quarry is directly back of the stone- 

 crusher, where the blending of the upper and lower portions 

 can be easily seen. The same columnar structure in converg- 

 ing groups is exposed to the north and south of the quarry 

 though much obscured by the soil and debris. 



That the upper and lower portions of this lava sheet belong 

 to one and the same mass of rock is shown not only by the 

 uninterrupted continuance of the rock from the lower mass 

 into the more nearly vertical upper columns, specially notice- 

 able in the quarry in Llewellyn Park, and to some extent in 

 John O'Eourke's quarry, but also by the mutual accommoda- 

 tion of the different sets of columns in the latter place, and the 

 fact that along what has been supposed by some to be a plane 

 of contact between a lower and upper flow, the columns which 

 should then have formed last are not perpendicular to that 

 plane, but meet it at all angles. 



The lower massive columns at the southern end of John 



