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 «, they are said to crop out, and the 
angle at which they incline below the horizon is 
their dip ; 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. 
