96 ELEMENTARY GEOLOGY 
is made up of concentric layers (Fig. 42) like an onion. 
Odlites appear to be formed in three distinct ways: 
(1) by the action of low forms of plants (alge), which 
build up the grains; (2) by chemical deposit in water 
at the surface (as in the geyser region of the Yel- 
Fig. 42. 
Section of oolite rock enlarged by the microscope. 
Several oélitic grains shown in cross-section. 
lowstone, where 
the balls grow to 
considerable size 
(Fig. 43) ; (8) by 
achemicalchange 
which causes a 
rock of a differ- 
ent origin to as- 
sume the condi- 
tion of an odlite. 
Odlite grains are 
even now ac- 
cumulating on 
many shores, as 
those of Florida 
andthe Great Salt 
Lake. 
Importance of Sedimentary Rocks. —The sedimen- 
tary rocks, particularly the mechanical and organic, are 
of the greatest importance to man, for they furnish 
him with most of his building-stones. 
At present such 
deposits are forming all over the globe, —on the land 
