CHAPTER V. 



STRUCTURES COMMON TO ALL ROCKS. 



WE have now given a brief account of all the different 

 kinds of rocks. But there are still some structures which 

 are found in all kinds of rocks, and which could not be 

 described until these kinds had been defined. These are: 

 1. Joints; 2. Great fissures; and, 3. Mineral veins. 

 Mountain-chains, as involving all kinds of rocks and all 

 kinds of structure in fact, as summing up all the prin- 

 ciples of dynamical and structural geology we must take 

 last of all. 



SECTION I. JOINTS AND FISSURES. 



Joints. 



We have already alluded to joints in stratified rocks 

 (page 179), but without describing them, because not 

 characteristic of these rocks. All rocks sedimentary, 

 igneous, and metamorphic are divided by cracks in dif- 

 ferent directions into separable blocks of various sizes and 

 shapes. These cracks are called joints. In stratified 

 rocks, one of the division-planes is between the strata, and 

 the other two nearly at right angles to this. The shape and 

 size of the blocks differ in different kinds of rocks. For 

 example, in sandstone the blocks are usually very large 

 and roughly prismatic ; in limestone, they are usually 

 very regularly cubic (Fig. 135) ; in shale, oblong rhom- 

 boidal ; in slate, small and sharply rhombic ; in granite, 

 sometimes large and roughly cubic, sometimes scaling in 

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