STRATIFICATION. 



59 



When they are not horizontal, they are said to 

 dip, and sometimes they stand vertically upon their 

 edges. 



Fig. 10. 



When strata protrude above the surface of the 

 ground, as at a, they are said to crop out, and the 

 angle at w^hich they incline below the horizon is 

 their dij) ; the thickness, of course, is the perpendic- 

 ular distance between their upper and under sur- 

 face. Some rocks, particularly slates, are subdi- 

 vided by numerous vertical joints, dividing the 

 whole mass into cubical or rhomboidal masses, as 

 in the following cut: 



Fig. 11. 



Outliers are strata detached from the main mass 

 of the bed to which they belong, as in the following 

 cut : 



Fig. 12. 



