506 



GEOLOGY. 



As found in the field, folds are usually much eroded, and often com- 

 pletely truncated (Fig. 398). The determination of anticlinal or syn- 



North 80° East 



30° 

 Fig. 399. 



h H 

 h H 



Fig. 400. 



Fig. 399. — Diagram to show how dip and strike are recorded. 

 Fig. 400. — Map record of dip and strike, showing syncHnal structure. 



clinal structure is then not based on topography, or even on such sec- 

 tions as shown in Figs. 394 to 397, for such sections are relatively rare. 



H h 



H f- 

 H I- 



Fig. 401. 



Fig. 402. 



Fig. 401. — Map record of dip and strike showing anticlinal structure. 

 Fig. 402. — The structure of the area shown in Fig. 401, in cross-section. 



The structure is determined by a careful record 

 of dips and strikes. On the field map, the record 

 may be made as shown in Figs. 399 to 401, where 

 the free ends of the lines with but one free end 

 point in the direction of dip, while the other 

 lines represent the directions of strike. Apply- 

 ing this method, the structure shown in Fig. 

 400 represents a syncline, and that in Fig. 401 

 an anticline. In cross-section, the structure 

 presented by Fig. 401 would appear as in Fig. 

 402. Fig. 403 shows a doubly plunging anticline; 

 that is, an anticline the axis of which dips down 

 at either end. Fig 404 shows a combination 

 of synclines and anticlines, and Fig. 405 a cross- 

 section along the line av of Fig. 404. The 

 outcrops of rock where the dip and strike 



> 



~t 



■V 



V 



H 



h 



■i 



h 



H 



h 



-1 



h 



A 



A 



^T< 



^iG. 403.— 



Map record 



of dip 



and strike 



showing 



plunging 



(dipping 



down at 



ends) anticline. 



