232 



DISPLACEMENTS OF STRATA 



Fig 



Clinometer. 



tion, and occasionally even overturning and inverting them. In 

 the comparatively small exposures of strata which may be seen 



D in ordinary sections in cliffs 

 and ravines, the rocks appear 

 to be simply inclined, and the 

 strata themselves to be nearly 

 straight. But when the struct- 

 ure is determined on a large 

 scale, it is found that this ap- 

 pearance is due to the limited 

 area visible in one view, and 

 that the apparently straight 

 beds are really portions of great 

 curves. Such curves are called 

 folds. 

 Dip. — The angle of inclination which a tilted stratum 

 makes with the plane of the horizon is called the dip, and 

 is measured in degrees. The line or direction of the dip is 

 the line of steepest inclination of the dipping bed, and is 

 expressed in terms of compass bearing. For example, a 

 stratum is said to have a dip of 15 to the northwest. The 

 angle of dip is measured by means of an instrument called 

 a clinometer, of which many kinds are in use. An excellent 

 form of this instrument is shown in the annexed figure. 

 AB is a long, rigid arm which is applied to the upper sur- 

 face of the stratum, the dip of which is to be measured. 

 A C, the shorter arm, carries a spirit level, and is pivoted at 

 A, allowing it to be raised or lowered. DE is a graduated 

 quadrant, on which may be read the angle included between 

 the two arms. The long arm is laid on the surface of the 

 inclined stratum, and the short arm raised, until the spirit 

 level shows that it is horizontal, and the angle is then read 

 off from the graduated quadrant. Figure 89 shows that 

 this is the angle of dip. AB is the surface of the dipping 

 bed, the line EBF is in the plane of the horizon, and the 

 line CAD is the horizontal line of the spirit level, parallel 



