CHANGES IN THE STRATIFIED ROCKS QTT 
they imitate the shapes of animals, and many have taken them 
for petrified substances. 
If the concretions are carefully examined, they will be found 
substantially like the enclosing rock, and often built around some 
foreign substance, such as a shell, pebble, or the stem of a plant. 
Their origin seems to be that of slow accumulation about these 
foreign centres, which serve as places where the cement of the 
rock is gathered in greater quantities than in the surrounding 
area (Fig. 158). 
It is as if by some force, which may be called the concretionary 
Fig. 157. 
A group of concretions about one-sixth natural size. 
force, the cement is deposited from the water with greater ease 
here than elsewhere. The process is somewhat analogous to the 
growth of acrystal. The same tendency for like substances to 
accumulate about centres, is seen in chalk and in other beds of 
carbonate of lime, where accumulations of silica are built up into 
concretionary nodules of flint. 
Joint Planes. — Jn Sedimentary Rocks. One of the 
secondary structures in the sedimentary rocks, is the 
breaking or cracking along planes, without any move- 
