Mesozoic Igneous rock of New Jersey. 329 



Such a parting is well developed in the large columns in 

 John O'Eourke's quarry, the major axis of the ellipsoid being 

 vertical, as it should be if the weight of the superincumbent 

 mass counteracted any resistance to contraction in a vertical 

 direction. The shells formed are often quite thin, being one or 

 two inches in width. This system of cracks is subsequent to 

 the columnar parting, each group of ellipsoidal sunderings be- 

 ing confined to a single column, and not extending into adja- 

 cent ones. The lower half of most of these spheroidal groups 

 appears to have been better developed than the upper half, but 

 this may result from the secondary action of frost which would 

 be greatest on the cracks retaining most moisture, as those 

 curved upwards naturally would. Occasionally the upper set 

 of cracks are as well marked as the lower and adjoin the lower 

 set of a spheroidal group above. Undoubtedly the spheroidal 

 sundering in this instance has resulted from the contraction of 

 a cooling mass of rock, but differs from the columnar parting 

 in that the latter is the result of progressive cooling from the 

 surface of a mass inward, while the former is produced by a 

 contraction acting equally in all directions through a mass. 



The cup-shaped jointing of columns so well developed in 

 some localities is probably a form of spheroidal sundering 

 brought out by a resistance to contraction along the length of 

 the columns and may curve in either direction as the result of 

 particular conditions, the minor axis of the ellipsoid lying in 

 the direction of maximum resistance. 



The wavy form of the columns large and small suggests 

 irregularities in the mass which disturbed the uniform advance 

 of the lines of maximum strain and caused them to deviate 

 from parallelism. 



The superficial banding of the large vertical columns by 

 nearly horizontal notches or grooves, which produce bands 

 three or four inches wide, frequently ends squarely in a way 

 which somewhat resembles layers of bricks, as suggested by 

 Professor George H. Cook, in his annual report on the geology 

 of New Jersey for 1884, where he describes the columnar 

 structure in John O'Rourke's quarry. This banding, however, 

 generally continues some distance without interruption, is not 

 always horizontal and sometimes curves abruptly, appearing 

 more like rude chiseling. The same structure is produced on 

 columns of dried starch, it is wholly superficial and seems to 

 be simply a modification of the plane of the crack. A cause for 

 its development has not yet suggested itself- 



The question of the origin and nature of columnar structure 

 in lavas and other substances has been discussed by Gr. Poulett 

 Scrope in his work on " Volcanos" (2d edition, London, 1872), 

 who by a somewhat different course of reasoning arrived at essen- 

 tially the same conclusions as those reached in the present paper. 



