CONTORTIONS OF THE STRATA 



47 



sure by use of a compass provided with a spirit level. When that 

 edge of the leveled compass which is parallel to the north-south 

 line upon the dial is held against the sloping rock stratum, the 



FIG. 27. A geological section based upon observations at outcrops, but with 

 the truncated arches restored. 



angle of strike is measured in degrees by the compass needle. If 

 the cardinal directions have been placed in their correct positions 

 upon the compass dial, the needle will point to the northwest 

 when the strike is northeast, and vice versa (Fig. 28 a). Upon 



FIG. 28. Diagram to illustrate the manner of determining the strike of rock beds 

 at an outcropping, a, a compass which has the cardinal directions in their 

 natural positions ; 6, a compass with the east and west initials reversed upon the 

 dial ; c, home-made clinometer in position to determine the dip. 



the geologist's compass it is therefore customary to reverse the 

 initials which represent the east and west directions, in order that 

 the correct strike may be read directly from the dial (Fig. 28 6). 



By the dip is meant the inclination of the stratum at any expo- 

 sure, and this must obviously be measured in a vertical plane 



