CHANGES IN THE STRATIFIED ROCKS 283 
pebbles, even those of tiny size, in preference to going — 
around the ends. ‘To explain this it is necessary 
merely to remember that when these joints were 
formed, there were layers of rock above them, which 
pressed down with so much weight that the easiest 
plane of breakage was the shortest, not the ragged, 
irregular crack, such as would result if the rock had 
been broken in the air, where the pressure upon its 
surface is so slight. 
Folding of Rocks. — Zerms Used. While many of 
the sedimentary strata have been elevated above the 
sea in nearly their original horizontal position, some 
have been moderately disturbed, and many, particu- 
larly those in mountains, have been tilted and folded 
until they stand on end in a nearly vertical position. 
Certain terms used by geologists in describing these 
folded rocks must be introduced here. Where strata 
are exposed to the air in a cliff or ledge, or any 
other natural exposure, they are said to crop out, 
and the rock is called an outcrop (Figs. 139, 167, etc.). 
If the stratum is more or less tilted, the direction in 
which the bed extends into the earth is called the dip 
(Fig. 163). This is the inclination of the surface of 
the layers, not necessarily of the rock face. It is as 
if we inclined a book away from us and considered 
the leaves to be strata. The dip in this case would 
be the inclination of the leaves, or of the covers. 
