56 



EARTH FEATURES AND THEIR MEANING 



FIG. 37. Diagram to show how sets of master joints 

 differing in direction by half a right angle may 

 abruptly replace each other. 



omnipresent double series or 

 set of joints is the well-known 

 set of master joints, and very 

 often it is found developed 

 practically alone (Fig. 36). 

 Over large areas, the direction 

 of the set of master joints 

 may remain practically con- 

 stant, or this set may quite 

 suddenly give place to a sim- 

 ilar set which is, however, 

 turned through half a right 

 angle from the first (Fig. 

 37). Not infrequently two 

 such sets of master joints 

 are found together bisecting 

 each other's angles within the 

 same rocks, and to them 



attitudes, they are, in 

 fact, seen to be inter- 

 sected by two parallel 

 series of fractures 

 which are perpendicu- 

 lar to the earth's sur- 

 face and to each other 

 and are described as 

 joints. In many cases 

 more than two series of 

 such fractures are 

 found, yet even in 

 these cases two more 

 perfectly developed 

 series are prominent 

 and almost exactly 

 perpendicular to each 

 other as well as to the 

 earth's surface. This 



FIG. 38. Diagram to show the different 

 combinations of the series composing two 

 double sets of master joints, and in a, a, a 

 additional disorderly fractures. 



