THE STRUCTURE OF THE EARTH 



259 



verticality in horizontal rocks, but are inclined at various angles 

 in strata which have been folded. In horizontal strata it is usually 

 found that two vertical systems of joints are present at right angles 

 to each other (Fig. I, p. 23), although more than two systems are often 

 present. Two remarkable features of the joints of undisturbed, 

 sedimentary rocks are (1) the horizontal extent of the joints which 

 stretch many hundreds of feet in some cases, and (2) the smoothness 

 of their faces. One often finds that the calcareous concretions and 

 even the quartz pebbles contained in some beds are broken in two 

 where joints cross them, with faces as smooth as if cut by a saw, and 

 the faces of joints exposed in cliffs 

 are often seen to be as smooth as 

 a plastered wall. The distance be- 

 tween joints varies from a fraction 

 of an inch to many yards. 



Joints extend to considerable 

 depths but cannot exist below the 

 zone of fracture. They frequently 

 end when a stratum of a different 

 character is reached ; for example, 

 a joint which extends through a 

 limestone may end when it reaches 

 a shale. In such case other joints 

 of a different interval may extend 

 through the lower stratum. 



Joints are taken advantage of in 

 quarrying, but if they are very 

 close together the blocks may be 

 too small for building purposes, 

 and if too far apart may make the 

 profitable quarrying of the rock 

 impossible. Some rich ore veins 

 are developed in joints (p. 370). 



Origin of Joints. — The origin of 

 joint planes in sedimentary rocks 



is not fully understood. It is generally believed, however, that they 

 are the result of movements of the earth's crust and have been pro- 

 duced by powerful mechanical stresses and strains which are the result 

 either (1) of torsion, or (2) of compression brought about by crustal 

 movement. A suggestive experiment (Fig. 250), in which plates of 



Fig. 250. — A plate of ice fractured by 

 twisting movement, the cracks produced 

 being chiefly nearly at right angles to 

 each other. Some joints seem to have 

 a similar origin. (After Daubree.) 



