STRUCTURAL GEOLOGY, 



499 



The successive flows are not usually coextensive. If the later flows 

 of the closing stages of a period of vulcanism fail to reach as far as the 

 earlier ones, a terraced or step-like aspect is given to the region, whence 

 the name trap-rock {trappe, steps) is derived. Such lava sheets, espe- 

 cially if of basalt, often assume a columnar structure in cooling, the 

 columns being rude six-sided prisms standing at right angles to the 

 cooUng surfaces (Figs. 379 and 380). This phenomenon is usually 

 best developed where the sheet is intruded between layers of preexist- 

 ing rock in the form of sills. The formation of the columns is some- 

 times regarded as a variety of concretionary action, but more commonly 

 as a result of contraction. The former is suggested by the ball-and- 

 socket ends of the sections of some columns (Fig. 382). The develop- 

 ment of the columns by contraction may be explained as follows: The 

 surface of the homogeneous lava contracts about equally in all direc- 

 tions on cooling. The contractile force may be thought of as centering 

 about equidistant points. About a given point, the least number of 

 cracks which will relieve the tension in all directions is three (Fig. 383). 

 If these radiate symmetrically from the point, the angle between any 

 two is 120°, the angle of the hexagonal prism. Similar radiating cracks 

 from other centers complete the columns (Fig. 384). A five-sided 

 column would arise from the failure of the cracks to develop about some 

 one of the points (Fig. 385). 



When lava is forced into crevices or rises to the surface through 

 fissures, and the residual portion solidifies in them, it gives rise to dikes, 

 as illustrated in Figs. 2 and 417 (not a true dike). Dikes are sometimes 



Fig. 381. Fig. 3S2. Fig. 383. 



Fig. 381. — Sections of columns from Giant's Causeway, coast of Ireland. 

 Fig. 382. — Ball-and-socket joints in cokunns of basalt. (Scrope.) 

 Fig. 383. — Diagram to illustrate the first stages in the formation of hexagonal columns 

 by contraction. 



