t^ 49S 



GEOLOGY. 



STRUCTURAL FEATURES OF IGNEOUS ROCKS. 



Certain 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 no internal lamination or 





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Fig. 380. — Columnar structure, obsidian cliff, Yellowstone Park. 

 (Iddings, U. S. Geol. Surv.) 



stratification, and the resulting rock is usually classified as massive 

 rather than stratified; but when a succession of flows occur, each indi- 

 vidual flow forms a layer, and the series as a whole becomes stratiform. 



