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first, that the stratum shall be sufficiently rigid to transmit a maximum 

 stress in a definite direction ; and second, that movement on bedding 

 planes shall afford a means to adjustment to curvature. It is an open 

 question how far these conditions are effective in a deep zone of flow. 

 In the opinion of the reviewer neither of them exercises a dominant 

 influence. Distinctions of stratification may become insignificant 

 under excessive pressure, and great friction on bedding planes may 

 prevent accommodation, and the mass may be deformed by dislocation 

 in the diagonal planes. This is illustrated in model J, Plate XCIII, 

 stages / to k, " Mechanics of Appalachian Structure." The accom- 

 modation of the soft material to shortening in one direction and 

 elongation in another is there accomplished by the reciprocal dis- 

 placement of wedges, which are defined by planes diagonal to the 

 applied force. This explanation is briefly suggested by Van Hise in a 

 subsequent paragraph on the depth of the zone affected by faults, but 

 he fails to emphasize the idea that while thrusts developed in the zone 

 of fracture pass into folds below, folds in turn may be replaced by 

 thrusts at still greater depths. The relation is important." 



Comparing the effect of few great structures with that of numerous 

 small ones, Van Hise concludes : 



" The average deformation of a region may be the same whether 

 it be by a few great faults with little or no fissility, by more frequent 

 lesser faults with or without fissility, by faults and overfolds with or 

 without both cleavage and fissility, or by folding with or without 

 faults and cleavage ; also that there is every gradation between faulting 

 and fissility, and probably every gradation between faulting and 

 cleavage." 



In Section VI, Van Hise discusses dynamic breccias and pseudo 

 conglomerates under the generic term of " autoclastic " rocks, which 

 was first used by H. L. Smyth. The development of autoclastic rocks 

 is limited to the zone of fracture and they are not of common occur- 

 rence in strata younger than the Cambrian. But where they occur in 

 rocks which have undergone repeated disturbances the resulting 

 dynamic breccia may so resemble a basal conglomerate as to lead to 

 an erroneous inference of unconformity. In the discussion of these 

 cases Van Hise inadvertently illustrates with what remarkable care and 

 patience the geologic study of the Marquette district has been pursued. 



Metamorphism is treated in sections VII and VIII, with the purpose 

 " briefly to sketch a few of the more important processes which -have 



