424 REVIEWS 



The two sets of folds may be produced simultaneously or at distinct 

 epochs by two or more thrusts in diagonal directions or at right 

 angles. Any two diverse thrusts inclined to each other may be resolved 

 into two forces at right angles, which will produce complex folds ; if, 

 however, the divergence of the two thrusts is moderate, their effect may 

 be combined to produce a simple system of parallel folds. 



Of the two thrusts producing complex folds, one is usually the 

 major thrust, the other the minor thrust, and the system of composite 

 folds to which one or the other gives rise may be described accordingly 

 as the major or the minor system. The length of canoes is propor- 

 tioned to the difference in power between the major and minor thrusts* 

 If the major thrust greatly exceeds the minor, the complex folds will 

 be long, narrow, and parallel ; if the two thrusts are nearly equal the 

 folds will be domes and basins. 



As an example of complex folds Van Hise refers to the Appalachian 

 region. The reference, perhaps, requires qualification to modify its 

 general application to the whole province. Districts of New England 

 and North Carolina do exhibit complex folds which are markedly 

 developed in the direction of the minor as well as of the major thrust, 

 but in the Appalachian Valley region from New York to Alabama the 

 folding scarcely appears to be complex. Van Hise cites the undula- 

 tory pitch of the axes as evidence of a pressure from northeast to 

 southwest. The pitch is to some degree due to variations of initial dip 

 and the depression of synclines in folding. There is no crenulation 

 of the major strikes such as might be expected to result from a trans- 

 verse thrust. If this zone of typical Appalachian structure shall be 

 shown to be complex in the sense in which Van Hise defines the term, 

 there will be good reason to assume that complex folding is the 

 general rule. This may probably be so. 



Van Hise's extensive experience in the study of complex structure 

 enables him to suggest practical methods as to the manner of determin- 

 ing whether the rocks of the district are complexly folded and in what 

 manner. These suggestions relate to the manner of making certain 

 necessary observations, which he sums up as folloAvs: • 



i. "Determine the strike and dip of the strata at the given point. 

 These give the resultant position of the strata as tilted by the force, 

 of folding in both directions." 



2. "Determine the direction of the pitch of the axes of the major 

 folds. The first is the direction of dip, and the second is the amount 



