STRATIFIED ROCKS. 



187 



This important subject must be taken up with some 

 detail, and for this purpose it becomes necessary to define 

 some common geological terms. 



Dip and Strike. The angle of inclination of strata 

 with the horizon is called the dip. There are always two 

 elements to be considered ; viz., direction and amount. 

 Thus a stratum may dip northward 30. The angle of dip 

 varies from to 90 i. e., from horizontality to verticality. 

 Sometimes strata are even pushed over beyond the vertical 

 such are called overturn-dips (Fig. 99). Examples are 

 found in all great mountain-chains, especially in the Alps. 

 When strata dip regularly, their thickness may be easily 

 estimated. For example, in walking from a to b (Fig. 

 100), we pass over strata whose thickness is Z c (= a ~b . sin 

 b a c). The dip may be accurately determined by means 

 of a clinometer (Fig. 101). 



FIG. 101. Clinometer. 



The direction of strata, or their line of intersection 

 with a horizontal plane, is called the strike. It is always 

 at right angles to the dip. If the dip is so many degrees 

 north or south, the strike will be east and west. If the 

 surface of the ground is level, the strike will be the same 

 as the outcrop, or appearance on the surface, of the 

 strata ; but this is seldom the case. If the strata are 

 plane, the strike will be a straight line. If the strata are 

 folded, the strike may be very sinuous (Fig. 107). In a 

 map view of strata, the dip and strike are represented by 

 the sign 1, in which the heavy line represents the strike, 



