MASTER JOINTS 



26$ 



pies might be mentioned. In many of the granites and other 

 coarse-grained igneous rocks, the joints are so arranged as to 

 divide the mass into cubical blocks, or into long, rectangular 

 prisms. In others, again, the blocks are of exceedingly irregular 

 form and size. 



In sedimentary rocks the joints are ordinarily in only two planes, 

 the third being given by the bedding planes. In homogeneous, 



FlG. 115. — Slip Rock, Juniata River, Pennsylvania. Steeply inclined strata, showins 

 joints. (Photograph by Rau.) 



heavily bedded sediments, such as limestones and massive sand- 

 stones, the joints are apt to form cubical or rectangular-prismatic 

 blocks, making a weathered cliff look like a gigantic wall of regular 

 masonry. Other sedimentary rocks are, as a rule, more irregularly 

 jointed. 



Joints are of very different orders of importance : some, the 

 master joints, traverse many strata and remain constant for long 

 distances and considerable depths, while each layer usually has 



