262 JOINTS 



into approximately parallel positions of random original particles" 

 (Van Hise). Cleavage is developed by a compressive force at 

 depths where the rocks are under sufficient weight of overlying 

 masses to be plastic, and therefore in the zone of flowage. 



Fissility is likewise due to compression, but in this case, the 

 rocks yield along the shearing planes, which are inclined and not 

 normal to the direction of pressure. Fissility arises in the zone 

 of fracture where the rocks are not loaded heavily enough to be 

 plastic ; but as the depth of this zone varies for rocks of different 

 degrees of rigidity, cleavage may be produced in a softer stratum 

 and fissility in a harder one at the same depth. 



Joints. — With the exception of loose incoherent masses, like 

 sand and clay, all rocks, however they may have been formed, are 

 divided into blocks of greater or less size by systems of cracks, 

 known as joints. These may be well observed in any stone-quarry, 

 because the art of quarrying consists in utilizing these natural 

 divisions of the rock. Were it not for them, quarrying would be 

 much more difficult and costly than it is. 



In the igneous rocks all the division planes which separate the 

 mass into blocks are true joints, and they vary much in the way 

 in which they intersect one another, and in the consequent shape 

 of the blocks. The fine-grained basaltic rocks display a very gen- 

 eral tendency to columnar jointing, forming more or less regular 

 prismatic columns, which are more frequently hexagonal than of 

 other shapes. In our study of modern volcanoes (p. 48) we 

 learned that certain modern lavas display these same characteristic 

 hexagonal columns and the ancient basalts of very many regions 

 have them. In certain cases, as in the famous Giant's Causeway, 

 of Ireland, the columns are divided transversely by concave joints, 

 giving a curious " ball and socket " structure, which almost seems 

 artificial. While regular hexagonal columns are much more fre- 

 quent in the fine-grained basalts, they are not confined to that 

 family. The acid glass of Obsidian Cliff, in the Yellowstone Park, 

 is jointed in the same fashion, though somewhat less regularly 

 (Fig. 15). Mato Tepee of South Dakota is a mass of phonolite, 

 jointed into magnificent columns (see Fig. 127), and other exam- 



