220 



STRUCTURAL GEOLOGY. 



are the result of outpouring on the bed of the sea, and 

 covering it with sediment,, then the igneous and the 

 stratified rocks are contemporaneous. Finally, if dikes 

 outcropping on the surface are covered with other strata 

 through which they do not break (Fig. 128), then they 

 are younger than the lower series, a, and older than the 

 upper, #. 



Some Structures common to Many Eruptives. 



Columnar Structure. Many eruptive rocks, espe- 

 cially of the more basic kinds, seem to be wholly made 



FIG. 129. Columnar basalt, New South Wales (Dana). 



up of regular prismatic columns (Fig, 129). This re- 

 markable structure is most common and perfect in basalt, 

 and is therefore often called basaltic structure. The col- 

 umns vary in size from a few inches to several feet in 

 diameter, and in length from a few feet to one hundred 

 feet ; the number of sides from three to seven, more com- 



FIG. 130. Basaltic columns (after Geikie). 



