STRUCTURAL GEOLOGY. 525 



be oblique to the direction of dip and strike {ef, Fig. 436), an oblique fault, 

 the outcrop of such a layer as H will have the relations shown in Fig. 441 

 if the downthrow was to the left, and that shown in Fig. 442 if the 

 downthrow was to the right. In the former case, it is said that there is 

 offset with overlap; in the latter, offset with gap. The amount of the 

 overlap and gap, respectively, increases with the increase of throw and 

 hade, and decreases with increase of dip. In all cases the outcrop (after 

 the degradation of the upthrow side) is shifted down dip. 



If a fault crosses folds at right angles to their axes, the effect is to 

 change the distance between the outcrops of a given bed on opposite 

 sides of the fault, after the truncation of the folded beds. The distance is 

 decreased on the upthrow side of a syncline (Fig. 443) and increased on 

 the upthrow side of an antichne (Fig. 444). If the throw of a fault in 

 tilted beds diminishes in one direction, it may cause beds to outcrop, 

 as shown in Fig. 445. Various other comphcations arise under other 

 circumstances. Since faults rarely show themselves in the topography 

 of the surface, except under special circumstances (see p. 151), their 

 detection and measurement is usually based on the study of the relations 

 of the beds involved, as illustrated by Figs. 436-445. 



