58 EARTH FEATURES AND THEIR MEANING 



joints may be such as to defy all attempts at orderly arrange- 

 ment. 



The space intervals of joints. The same kind of subequal spac- 

 ing which characterizes the fractures near the surface of the block 

 in DaubreVs experiment (Fig. 19, p. 41) is found simulated by the 

 rock joints (Fig. 39). Such unit intervals between fractures may 

 be grouped together into larger units which are separated by frac- 

 tures of unusual perfection. We may think of such larger space 

 units as having the smaller ones superimposed upon them (Fig. 40). 



The displacements upon joints faults. In the vast majority 

 of cases, the joint fractures when carefully examined betray no 

 evidence of any appreciable movement of the two walls upon each 

 other. Generally the rock layers are seen to cross the joints with- 

 out apparent displacement. Joints are therefore planes of dis- 

 junction only, and not planes of displacement. 



Within many districts, however, a displacement may be seen 

 to have occurred upon certain of the joint plunes, and these are 

 then described as faults. Such displacements of necessity imply 



FIG. 41. Faulted blocks ef basalt divided by joints near Woodbury, Connecticut. 

 To show the structure of the rock, some of the foliage has been removed in prepar- 

 ing the sketch from a photograph. 



a differential movement of sections or blocks of the earth's crust, 

 the so-called orographic blocks, which are bounded by the joint 

 planes and play individual roles in the movement. A simple case 

 of such displacements in rocks intersected by a single set of mas- 

 ter joints is represented in the model of plate 4 C. The most promi- 

 nent fault represented by this model runs lengthwise through the 

 middle, and the displacement which is measured upon it not only 

 varies between wide limits, but is marked by abrupt change's at 

 the margins of the larger blocks. This vertical displacement upon 



