SUGGESTIONS FOR TEACHING 



183 



Figure 13. — Beds repeated at Surface 



Faults might be introduced in order to make the conception of a fault 

 more accurate and precise. 



With a more advanced class the local displacements in the different 

 parts of the fault should be described and their characteristics discussed, 

 beginning with a dip-slip fault (the simplest type of fault and fault- 

 movement), all of whose elements can be shown in a vertical section 

 taken at right angles to the strike of the fault and of the strata. With 

 the help of figures 9, 10, 11, 

 and 12, which are sections of 

 this kind, the various ele- 

 ments of the displacement 

 should be denned, and the re- 

 lations between them, as given 

 below the figures, should be 

 brought out. The character- 

 istics of vertical, normal, and 

 reverse faults should be 

 pointed out, and il should be shown that the outcrops of the strata will be 

 repeated on opposite sides of the fault if the strata dip toward the up- 

 throw side of the fault, and that some strata will not outcrop at all if 

 the strata dip toward the downthrow side of the fault. This may be 

 illustrated by figures 13 and 14, which represent vertical faults. It is 

 quite evident that if the fault should dip to the right we should have a 

 reverse fault, and if to the left a normal fault. There would be no 



change at the surface if 

 the fault-line were not 

 moved. Unless the dip of 

 the fault can be found, 

 surface observations will in 

 many cases not decide 

 whether the fault is nor- 

 mal, vertical, or reverse. 



Strike, dip, and oblique 

 fa nils should now be de- 

 fined and an oblique fault considered as the general case of all three. 

 Figures 15, 16, and 17 show the displacements in plan and in two sec- 

 tions. It is necessary here to define apparent displacements and to do- 

 scribe the relations of the two branches of the outcrop as dependent on 

 the* directions of the strike of (he fault and of the strata, defining also 

 the gap and overlap.-' 5 



Figure 14. — Beds missing at Surface 



23 These relations are well Illustrated in Chamberlln and Salisbury's Geology, 



