264 JOINTS 



minor joints which are confined to that bed. One set of joints, 

 the strike joints, run more or less parallel to the strike of the 

 beds, while the second set, the dip joints, follow the dip ; the 

 former are usually the longer and more conspicuous. 



Cause of Joints. — With regard to the manner of their produc- 

 tion, joints maybe classified into two series : (1) those which are 

 due to tension, the rock usually parting in planes normal to the 

 directions of tension; (2) those which are due to compression, 

 the cracks forming in the shearing planes. 



( 1 ) Tension Joints. — In igneous rocks joints are caused by the 

 cooling and consequent contraction of the highly heated mass. This 

 shrinkage sets up tensile stresses in the mass to which the rock yields 

 by cracking and parting, the shape of the blocks being largely con- 

 trolled by the coarseness or fineness of the mass. In some cases the 

 jointing of sedimentary rocks may perhaps be caused by a shrinkage 

 of the mass on drying, but this cannot be an important method of 

 producing systems of joints. 



The convex sides of anticlinal and synclinal folds are stretched, 

 and (provided they are not too deeply buried) the stretching may 

 result in a system of cracks radial to the curves which follow the 

 strike of the beds. Folds are not horizontal, but pitch in the 

 direction of their axes. This complex folding may produce two 

 sets of tensile stresses perpendicular to each other, and thus cause 

 two series of joints, one following the strike and the other the dip 

 of the beds. Complex folding must produce a twisting and warp- 

 ing of the strata, and it has been experimentally shown that a 

 brittle substance when twisted cracks in two sets of fractures which 

 intersect nearly at right angles. How slight is the twisting and 

 warping needful to produce joints is shown by the fact that strata 

 which are perfectly horizontal, so far as can be detected, are 

 jointed, as are also certain modern coral limestones. 



Tension joints produce either rough, or smooth and sharply 

 cut surfaces, which is determined by the character of the rock. 

 In sandstones which are weakly cemented the cracks pass 

 between the grains, while in hard and firm rocks the fractures 

 are clean. 



