STRUCTURE AND POSITION. 



177 



tion of dip by a compass. These two are often conveniently muted in 

 one instrument. As thus determined the angle of dip varies from 

 0° to 90°, or from horizon- 

 tally to vertically. Fig. 

 150 is an example of verti- 

 cal strata. In strongly 

 folded rocks the strata 

 may be pushed beyond the 

 perpendicular (Fig 151), 

 or even completely re- 

 versed (Fig. 205(7), so that 

 the change from the original position may be nearly or quite 180°. 



In a series of regularly dipping strata, like that of Fig. 148, it is easy 

 to estimate the thickness of the series. The thickness, l c, = distance 



Fig. 148. 



Fig. 149.— Clinometer. 



ah X sin. of the angle of dip 30°. We sometimes find an actual sec- 

 tion such as that represented in the figure, but more usually we observe 

 the successive outcrops on 

 the surface and the angle 

 of dip, and construct an 

 ideal section. This is easy 

 enough if the rocks are 

 bare, but if covered with 

 soil, we must take advan- 

 tage of every bare spot, of 

 every ravine, gully, and 

 stream - bed where the 

 rocks may be exposed, of 

 every quarry, railroad-cut- 

 ting, well, etc., and put these together in the attempt to make a map of 

 outcrop and a section. 



The strike of strata is the direction of their trend, or, more accu- 

 rately, is the line of intersection of the strata with a horizontal plane. 

 It is always at right angles to the line of dip. If the dip is north or 

 south, the strike is east and west. If the strata are plane, the strike is 

 a straight line ; but if they are bent, the strike may be a curve. For 

 12 



Fig. 150.— Vertical Strata. 



