70 



LYELL'S ELEMENTS OF GEOLOGY. 



Dip and Strike. 



avoid being occasionally deceived, both as to the point of dip and 

 the amount of it. 



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 

 cliflf, 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. Perfectly horizontal lines 

 in the face of a vertical cliff may be the edges of highly inclined 

 strata, if the observer see them in the line of their strike, their 

 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. 64.), we may sup- 



Fig. 64. 



Apparent horizontality of inclined strata. 



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

 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 endeavour to find a ledge or portion of the 

 plane of one of the beds projecting beyond 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 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. 65. It is thus easy to discover 

 whether the lines of the inclined beds bisect the angle of 90°, 



