26 C. R. VAN HISE 



It, is apparent that the Paleozoic strata of the Mississippi 

 valley and of the Plateau region, the Alleghenies, and Coast 

 Ranges were folded under such conditions that the curves of 

 the folds were produced not by actual bending of the layers, but 



\ 

 \ 



Fig. 9. — Diagram of radial openings produced by tensile fracture. 



by numerous fractures, with a slight displacement of each block, 

 resulting in a curved form (Figs. 9 and 10). 



Now these joints must have been produced by tensile 

 stresses or by shearing stresses. If they are of the first class, 

 it is self-evident that the production of the joints involved 

 surficial elongation (Fig. 9). If they are of the second class, 

 their production may have involved all of the surficial elonga- 

 tion (Fig. 10), and it will be explained in a subsequent number 

 of this Journal 1 that joints of this kind are believed to be wide- 

 spread. Some reasons for this belief may here be mentioned. 

 These joints in many regions show a marked tendency to a 

 vertical attitude, as in figure 10. Also the kind of displacements 

 generalized in figure 10 has been observed at various places. 

 Moreover, such joints are closer together the closer the folding, 

 and in some cases they are so close as to make the intervening 

 masses approach leaflets, as, for instance, in sandstones and 

 shales on the Chesapeake and Ohio canal, three miles west of 

 Hancock, Md. 



In both the cases of joints produced by tension and shear- 

 ing above described, there is no real elongation of the strata, 

 but merely a displacement of the blocks causing surficial 

 elongation. In the case of the tension joints this elongation 

 is due to the fact that spaces are measured ; in the case of the 



'Deformation of rocks, by C. R. Van Hise : Journ. Geol., Vol. VI, 1898. 



