70 PHYSICAL GEOLOGY 



line made where the inclined stratum is cut by a horizontal 

 plane; and it will be seen that it js always at right angles to 

 the direction of the dip of the same beds. 



Obviously a fold cannot be indefinitely long. It dies 

 out at either end, or in other words, the axis itself is gently 

 arched and slants downwards in opposite directions from the 

 highest point of the fold (Fig. 51). This slant of the axis is 

 called the pitch of the fold and is to be carefully distinguished 

 from the dip of the beds. In a circular dome the angle of 

 pitch is the same as that of the dip of the strata, but in all 

 other folds it is less, except along the axis. Among synclines 

 the features are much the same except that the pitch and dip 

 are directed inward toward the bottom of the trough and the 

 order of succession of the strata, as they outcrop on the sur- 

 face, is reversed. 



FRACTURES 



Joints. The uneven risings and sinkings of the crust 

 tend to crack the brittle strata in every direction. Certain 

 rocks, such as drying mud and cooling lava, crack also be- 

 cause of contraction. Every quarry and outcrop shows 

 parallel systems of cracks, usually upright where the rocks 

 have not been folded, but in many other places slanting. 

 These cracks are known as joints (Fig. 52), because of their 

 rude resemblance in some instances to the joints between the 

 blocks in a stone wall. 



Fissility. When the cracks are closely spaced and parallel 

 to each other, the rock breaks readily into plates. Slates 

 and other rocks in which such fracturing prevails are said to be 

 fissile (Fig. 53). Almost any rock may be either fissile or jointed, 

 or both, according to the conditions to which it has been sub- 

 jected. These conditions will be dealt with in later pages. 



Normal faults. Cracks thus divide the rocks into a 

 multitude of blocks of various sizes. These blocks are some- 

 times tilted, shifted, or let down, so that the ends of the broken 

 strata no longer match. Such dislocations are called faults. 



