STRUCTURAL GEOLOGY 



399 



when the unconformity may be slight. Unconformities are often 

 of great significance in the interpretation of geological history. 



Fig. 307. One phase of unconformity. The beds at the right were tilted 

 to their present position before the deposition of the beds at the left. 



There may be unconformities between stratified rock and igneous 

 rock, and between stratified rock and metamorphic rock, as well 

 as between different series of bedded rocks. 



Fig. 308. Columns of basalt (a variety of 

 igneous rock). The Giant's Causeway, 

 Ireland. 



Igneous Rocks 



Certain large structural features of igneous rocks have been 

 mentioned in treating of their origin in the previous chapter. 

 When a great flow of lava spreads out upon the surface, there is 



