MINOR STRUCTURES OF VOLCANICS 319 



13:^5.) Though most common in basic rocks, this structure is 

 also found in the acid types, as is well shown by the columnar 

 obsidian and other rhyolites of the Yellowstone National Park. 

 (Iddings-i8.) (Fig. 58.) 



Fig. 58. Columnar structure in obsidian. Obsidian cliff, Yellowstone Na- 

 tional Park. (After Iddings.) 



Variation in Grain. In both effusive and intrusive masses a 

 variation in the coarseness of texture or size of grain is observable 

 between the outer faces of the mass and its interior. In general 

 there is a regular increase in coarseness toward the center, due to 

 less rapid chilling of the inner portion of the mass. Queneau (22) 

 has been able to establish a table for determining distance from 



