ESTIMATES AND CAUSES OF CRUSTAL SHORTENING 25 



tion. However, the plunge of this anticline varies in passing 

 from north to south. In other words, there is here a great but 

 very gentle cross fold, and corresponding to this is another set 

 of joints which run in an east-west direction. 



Fig. 8. — Tensile joints. 



The same arrangement of joints is still better illustrated in 

 the closely folded Allegheny mountains, and the Coast Ranges 

 of Oregon and California. In the Allegheny mountains, as may 

 be seen by sections along the railroads (for instance, the Penn- 

 sylvania, and Baltimore and Ohio) in the stronger beds there 

 are two sets of joints everywhere corresponding to the strike 

 and dip, in other words, corresponding to the two directions of 

 fracture due to longitudinal and transverse folding. 



Such joints may be seen both in anticlines and synclines. 

 They occur in sandstone, grit, or limestone. Where the layers 

 are a foot or more in thickness, and the rocks are gently folded, 

 the joints may be several feet apart. Where the layers are closely 

 folded the joints are frequently less than a foot apart. In the 

 thinner layers, those from two to six inches in thickness, the 

 joints are ordinarily less than a foot apart, and where closely 

 folded are but two or three inches apart. Indeed, in some cases 

 of close folding, the two sets of joints are so close together as 

 to break the formations into a set of parallelopiped blocks, the 

 dimension along the bedding being the least of the three, that is, 

 the joints are closer together than the thickness of the beds. 



