192 STUDIES FOR STUDENTS 



age, and the second is a phenomenon of the zone of fracture. 

 The special point to which I wish here to call attention is that as a 

 result of the displacements of jointing, the strata may appear 

 in generalized curves of anticlinal and synclinal character 

 (Fig. 6). In various districts rocks which have not been so 



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Fig. 6. 



deeply buried as to be in the zone of flowage appear to be in 

 anticlines and synclines. In these cases it is believed that 

 the phenomena are explained as above suggested. This princi- 

 ple is illustrated by the slightly undulating rocks of the upper 

 Mississippi valley. Cutting these everywhere, and in many 

 districts in two directions at right angles to each other, are sys- 

 tems of joints. These joints are phenomena of fracture, and the 

 slight bowing, which one might represent as a fold, really is not a 

 fold in the sense of the deformation of the strata by flowage, but 

 is bowing as a result of very slight but abrupt changes in direc- 

 tion at the numerous joints, the general effect being to produce 

 a folded appearance. 



The apparent bowing due largely to jointing, so well illus- 

 trated in the Mississippi valley, is still more finely illustrated 

 by the Allegheny Mountains. The limestones and sandstones of 

 this mountain system, ordinarily regarded as deformed mainly 

 by folding, are largely deformed by jointing. If the course of 

 the strata be roughly platted, they" will appear to be in continu- 

 ous undulating curves. However, the rocks are everywhere cut 

 by two intersecting sets of joints at right angles to each other, 

 and it appears to be the case that the curved deformation is 

 really not mainly that of folding, but mainly that of fracture 



