54 



DIP AND STKIKE. 



[Ch. V. 



If the upper surface of a hard stony stratum be uncovered, whether 

 artificially in a quarry, or by the waves at the foot of a cliff, it is easy 

 to determine towards what point of the compass the slope is steepest, or 

 in what direction water would flow, if poured upon it. This is the true 

 dip. But the edges of highly inclined strata may give rise to perfectly 

 horizontal lines in the face of a vertical cliff, if the observer see the 

 strata in the line of their strike, the dip being inwards from the face of 

 the cliff. If, however, we come to a break in the cliff, which exhibits a 

 section exactly at right angles to the line of the strike, we are then able 

 to ascertain the true dip. In the annexed drawing (fig. 69), we may 

 suppose a headland, one side of which faces to the north, where the 



Apparent horizontality of inclined strata. 



beds would appear perfectly horizontal to a person in the boat ; while in 

 the other side facing the west, the true dip would be seen by the person 

 on shore to be at an angle of 40°. If, therefore, our observations are 

 confined to a vertical precipice facing in one direction, we must endeavor 

 to find a ledge or portion of the plane of one of the beds projecting be- 

 yond the others, in order to ascertain the true dip. 



It is rarely important to determine the angle of inclination with such 

 minuteness as to require the aid of the instrument called a clinometer. 

 "We may measure the angle within a few degrees by standing exactly 

 Fi g . 7o. opposite to a cliff where the true dip is 



exhibited, holding the hands immediately 

 before the eyes, and placing the fingers of 

 one in a perpendicular, and of the other in 

 a horizontal position, as in fig. 70. It is 

 thus easy to discover whether the lines of 

 the inclined beds bisect the angle of 90°, 

 formed by the meeting of the hands, so as 

 to give an angle of 45°, or whether it 

 would divide the space into two equal or 

 unequal portions. The upper dotted line 

 may express a stratum dipping to the north ; but should the beds dip 

 precisely to the opposite point of the compass as in the lower dotted 



