262 



DYNAMICAL GEOLOGY. 



The vertical position shows that the cooling surfaces were (1) the rocks 

 ■underneath and (2) the air above ; and the regularity of position indicates 

 remarkably equable progress through the mass in the cooling. Fig. 219, 

 representing a dike and overflow from the same region, shows the effects 

 of position in cooling surfaces ; the dike, with vertical walls having Jiorizon- 

 tal columns, and the overflow, vertical columns. The rock intersected and 

 overlaid is a conglomerate, and part of the latter is involved in the basalt. 



219. 



220. 



Dike with outflow, Kiama. D 



Curved columns, Kiama. D. '49. 



The effect of a small ridge (of conglomerate) in making curved columns 

 is shown in Fig. 220. For a short distance the basalt is massive ; then the 

 columns — one to four feet in diameter and 30 long — begin abruptly. The 

 low terminal plane of the column is flat ; but tliis plane is nearly Jiorizontal 

 whatever the obliquity of the prism, the variation from it where greatest iiot 

 exceeding 20°. The stream of basalt was 50 feet thick. 







Columnar Basalt, Orange, N.J. Iddings, '86. 



The trap of the Triassic of eastern ISTorth America is usually more or 

 less columnar; and in some places regularly so. At a quarry in Orange, 

 IS". J.,, west of the city, the columns are in groups which are in some parts 



