92 ELEMENTARY GEOLOGY 
large marine and land animals have been accumulated 
into bone beds. These form phosphate rocks which are 
used as fertilizers. Guano, which is found on islands 
off the coast of Chile, is made of the excrement of 
birds that lived in great numbers on these islands. 
Fig. 38. 
A limestone rock polished as a marble, so that sections of shells (Brackinp es 
show with distinctness. 
Plant Deposits. In the sea, plants do not exist in 
sufficient abundance to make beds of rock, although 
they are of aid in the formation of some strata. On 
the land, plants take carbon from the air and mineral 
