REVIEWS 421 



and flowage intermediate between an upper zone, in which all rocks 

 fracture, and a lower zone in which all flow. 



Gilbert and Dutton, as well as Heim, had anticipated the conclu- 

 sions of Van Hise in the general recognition of a zone of fracture and 

 a zone of flow, but Van Hise first describes clearly the phenomena of a 

 zone of combined fracture and flow, with which geological observation 

 of structure has chiefly to deal. He contributes also a closer analysis 

 of the phenomena in the uppermost and lower 'zones. The develop- 

 ment of autoclastic breccias, the production of folds by opening of 

 radial joints due to fracture, the association of deformation by fracture 

 and without fracture in the same zone, the adjustments of form by which 

 strata yield to folding, the disappearance of folds in depth, and finally 

 the phenomena in the zone of flowage, are clearly and critically dis- 

 cussed. 



The analysis of folds is based upon a classification by de Margerie 

 and Heim, published in 1888, but their ideas are amplified, and a com- 

 prehensive classification capable of including all types is developed by 

 the author's recognition of the complex nature of folds. The author's 

 primary classification divides flexures into simple, composite, and com- 

 plex folds. Simple folds are single anticlines and synclines, which 

 are subclassified in the usual manner as upright, overturned,, or recum- 

 bent, and as symmetrical or unsymmetrical folds. But Van Hise 

 recognizes that every fold of any magnitude is complicated by minor 

 folds and each of these again by still smaller folds, and so on to 

 microscopic plications of the ;/th order. When thus considered, the 

 major fold may be termed composite. Composite folds of the first 

 magnitude were described by Dana as geosynclines and geoanticlines, 

 and these, as Van Hise points out, are due to combined action of 

 gravity and lateral thrust. In the discussion of composite folds they 

 are classified as normal and abnormal, and the basis of classification is 

 the direction of convergence or divergence of the axial planes of the 

 secondary folds. Thus in a normal anticlinorium the axial planes of 

 the secondary folds converge downward and diverge upward, whereas 

 in an abnormal anticlinorium the axial planes of the secondary folds 

 converge upward and diverge downward. No exception can be taken 

 to this classification as a matter of fact, but as a scheme adapted to 

 instruction or to practical discussion it may not be generally useful, 

 because the mind is speedily confused by the repetition of the terms 

 of upward and downward convergence as applied to anticlinoria and 



