278 UNSTRATIFIED ROCKS 



or have thrust themselves between strata, following the path of 

 least resistance. Intrusions are younger, it may be vastly so, than 

 the strata which they penetrate and lie over or beneath; their 

 geological date may be determined by a process of elimination, 

 finding the newest strata which they have traversed and the oldest 

 which they have not reached. 



Different names are given to these subterranean masses, in 

 accordance with their shape, size, and relation to the strata with 

 which they are associated. 



Dikes. — A dike is a vertical or steeply inclined wall of igneous 

 rock which was forced up into a fissure when molten and there 



FIG. 122. — Diagram of dike. 



consolidated. Dikes of a certain kind may actually be seen in 

 the making, as when the lava column of a volcano bursts its way 

 through fissures in the cone. The ordinary dike is formed in fis- 

 sures which traverse stratified rocks, or, sometimes, cuts through 

 older and already consolidated igneous rocks. In thickness dikes 

 vary from a foot to a hundred feet or more, and pursue nearly 

 straight courses, it may be for many miles. The rock of a dike has 

 usually a compact texture, having cooled more slowly than the 

 volcanic masses, though the edges, chilled by contact with the walls 

 of the fissure, may be glassy. If the rock displays columnar joint- 

 ing, the prisms are horizontal, normal to the cooling surfaces. 



