REVIEWS 423 



"Folds thus produced by upward differential movement may be 

 called 'overthrust folds.' The axial planes dip toward the effective 

 stress, hence overthrust folds are those in which the axial planes dip 

 toward the force producing them. While the development of overthrust 

 folds is the general law, it may not infrequently happen that under 

 favorable conditions the beds or formations may be thrust forward and 

 downward. The folds thus produced by downward differential move- 

 ment may be called 'underthrust folds.' The axial planes dip away 

 from the effective stress, hence underthrust folds are those in which the 

 axial planes dip away from the force producing them.' 1 '' 



The idea of Rogers that deformation proceeded by a wavelike 

 motion, which, in the case of the Appalachians, emanated from the 

 Atlantic region, is firmly fixed in geology without being sufficiently 

 justified by facts. Appalachian structure in many districts presents a 

 monoclinal inclination toward the southeast. If it be overthrust, the 

 effective force operated from that direction. If it be underthrust, the 

 effective force acted from the northwest. There is nothing in the 

 monoclinal relations of folds which will determine that alternative. 

 Theoretically, the geologist is at liberty to infer overthrust in prefer- 

 ence to underthrust or vice versa, but an hypothesis of the origin of 

 force based upon that inference is of no value. The analysis given 

 by Van Hise leads him to the conclusion that the monoclinal folds 

 and cleavage which prevail in the crystalline areas of the Appalachians 

 are due to an effective force from the southeast, but that conclusion 

 cannot be considered as demonstrated. Indeed, other geologists 

 approaching the problem from a different point of view are justified in 

 reaching the reversed conclusion. 



It may be suggested that compression, which causes strata to fold, 

 is a force of gradual accumulation, that is, of such slow growth that 

 the pressure is transmitted through a rock-mass of indefinite extent 

 until movement ensues. Such movement may probably begin at a 

 point determined by local conditions, and may continue in a direc- 

 tion determined by the least resistance either as an overthrust or 

 an under-thrust. In that view the more effective force is simply 

 that which is opposed to the less effective resistance, and the hypoth- 

 esis of a wave -like influence emanating from a distant source is 

 unnecessary. 



Complex folds are defined by Van Hise as those canoes, basins, 

 and domes which result where two sets of folds intersect each other. 



