STRUCTURAL GEOLOGY. 



501 



whether formed by water or by lava-flows, is normally horizontal, or 

 nearly so. Both kinds of deposits, however, occasionally take place 

 on considerable slopes. Modifications of the original attitude result 

 from earth movements, and the measurement of these modifications 

 is an important feature of field study. It is recorded in terms of di'p 



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7' 





Fig. 386. — Diagram illustrating dip and strike. (Geikie.) 



and strike. The dip is the inchnation of the beds referred to a horizontal 

 plane, as illustrated in Fig. 386, and is usually measured by a clinometer, 

 the principle of which is shown in Fig. 387. In measuring the dip, the 

 maximum angle is always taken. In Fig. 386, for example, the angle 

 would be less if the direction were either to the right or left of that indi- 

 cated by the arrow. The direction as well as the amount of the dip is 

 always to be noted. This must be determined by the compass, to which 

 the clinometer may be conveniently attached. Dip 40°, S. 20° W. gives 

 the full record of the position of the bed of rock under consideration. 

 The strike is the direction of the horizontal edge of dipping beds, or 



Fig. 387.— The clinometer. 



more generally, the direction of a horizontal line on the surface of the 

 beds. This is illustrated in Fig. 386. Since the strike is always at 

 right angles to the dip, the strike need not be recorded if the direction 

 of the dip is. Thus dip 40°, S. 20° W. is the same as dip 40°, strike N. 

 70° W. 



