406 



GEOLOGY 



Joints. The surface rocks of the earth are almost universally 

 traversed by deep cracks called joints (Figs. 2 and 327). In most 

 regions there are at least two systems of joints, the crevices of each 



Fig. 326. Contorted shales and greywacke. Ardwell, near Girvan, Ayr- 

 shire. (H. M. Geol. Surv.) 



system being roughly parallel to one another, while those of the two 

 systems, where there are two, are approximately at right angles. In 

 regions of great disturbance, the number of sets of joints is often 



Fig. 327. Jointed rocks. Cayuga Lake, N. Y. (Hall.) 



three, four, or even more. The joints of each set may be many 

 yards apart, or in exceptional cases, but a few inches, or even a 

 fraction of one inch. 



