6l8 STUDIES FOR STUDENTS 



Since a fault may be no more than a displaced joint, this relation 

 is easily explained in the same manner as in the case of joints. 

 (See pp. 610-612). 



RELATIONS OF FOLDS TO THRUST FAULTS. 



It has been long recognized that thrust faults are often related 

 to overfolds. If the strata are in the zone of combined fracture 

 and flowage, the overfolds may be broken along the reversed 

 limbs and the arch limbs be thrust over the trough limbs. In a 

 region of overfolds and thrust faults, if it could be determined 

 whether the differential movements are such as to carry the 

 material moved toward the surface or away from the surface, it 

 could be decided whether such folds and faults should be called 

 overthrusts or underthrusts.' But the differential movements, 

 the forms of inclined and overturned folds, and the character of 

 the thrusts are identical, whether a given bed above be consid- 

 ered as moving forward and upward as compared with the layer 

 below, or be considered as moving forward and downward as 

 compared with the layer above. In Fig. 16, if the force be con- 

 sidered as applied at A, it would be called an overthrust fault ; 

 if the force be considered as applied at B, it would be called an 

 underthrust fault ; and yet the phenomena are identical. The 

 movements must be such that the material goes in the direction 

 of relief, and it is probable that this is more often toward the 

 surface of the earth (see pp. 338, 339) rather than deeper within 

 the earth. It is probable that in certain cases thrust has been 

 transmitted by a strong formation or series and pushed under 

 other strata. This is particularly likely to occur where the lower 

 strata are weaker or where the material in advance of the active 

 strata transmitting the force has been already raised into folds, 

 and thus partly escapes the pressure. (See pp. 316-319, and 

 Fig. 6.) 



As explained by Willis, in regions which are but lightly 

 loaded the forces producing thrust faults may result in clean-cut 



'The term underthrust is taken from Professor E. A. Smith. Am. Jour. Sci., 

 3d series, Vol. XLV, 1893, pp. 305, 306 ; see also this Journal, Vol. IV, p. 339. 



