Van Rise and Hoshins — Pre-Camhrian Geology, 207 



zone of iiowage is at a different depth for two rocks of unequal 

 strength and for the same rock under different conditions dur- 

 ing deformation, there is a zone of combined fracture and 

 flowage. In a set of heterogeneous beds, upper, weak strata 

 may be in the zone of folding, while lower and stronger strata 

 may be in the zone of fracture. It is believed that the zone of 

 combined fracture and iiowage is probably 5,000 meters thick. 

 The three zones of fracturing, of fracturing and flowage, and 

 of flowage grade into one another. 



Analysis of Folds. — As ordinarily treated, folds are con- 

 sidered as simple flexures in two dimensions. As they occur 

 in nature, many folds are complex flexures in three dimensions. 

 Flexures in two dimensions may be simple or composite, and 

 the folds may be called simple folds or composite folds. Flex- 

 ures in three dimensions may be called complex folds. Fol- 

 lowing Margerie and Heim, simple folds are classified into 

 upright, inclined, overturned, and recumbent. Any one of 

 these folds may be ordinary, isoclinal, or fan-shaped. 



The greatest composite folds of the earth's crust, following 

 Dana, are called geanticlines and geosynclines. In the forma- 

 tion of these folds gravity is the predominant force, whereas 

 in ordinary composite folds thrust is an important or predomi- 

 nant force, — although in all folds thrust and gravity work 

 together. Composite folds are divided into normal and ab- 

 normal. Both normal and abnormal folds are divided into 

 upright, inclined, and overturned anticlinoria and synclinoria. 

 In normal anticlinoria the axial planes of the secondary folds 

 on opposite sides of the crest converge downward. In normal 

 synclinoria the axial planes of the secondary folds on opposite 

 sides of the trough diverge downward. In abnormal anticli- 

 noria the axial planes on opposite sides of the crest diverge 

 downward, and in abnormal synclinoria the axial planes on 

 opposite sides of the trough converge downward. It is shown 

 in the case of normal folds that, as a result of the action of the 

 forces, accompanied with normal adjustment, the axial planes 

 of the secondary folds have the attitudes above given. In the 

 case of abnormal folds the readjustment is concentrated along 

 certain zones, and the axial planes are rotated from their ordi- 

 nary position to the positions above given. In case of axial 

 planes of a set of folds in a mountain system inclined in the 

 same direction the folds are called monoclinal. Overthrust 

 monoclinal folds are defined as those in which the axial planes 

 dip toward the forces producing them, and underthrust folds 

 are defined as those in which the axial planes dip away from 

 the forces producing them. In the majority of cases it is held 

 that monoclinal folds are overthrust folds. 



If horizontal compressive forces are sufficiently strong to 



