SEDIMENTARY AND METAMORPHIC ROCKS 89 
so the student can hardly be expected to decide which 
are chemical, which organic, and which mechanical. 
There are, however, no chemical forms of conglom- 
erates, sandstones, or clays, but only rocks like those 
described in this section. Moreover, since they are 
accumulated under conditions of a peculiar nature, 
there is a general absence of fossil remains. In a dead 
sea, for instance, animal and plant life are nearly absent. 
Organic Rocks. — Calcareous Rocks. Both animals 
and plants are engaged in making strata. Animals on 
the land do not usually have an opportunity, because 
they do not live in great colonies, and when they die 
their bodies soon decay. In the ocean, on the other 
hand, there are reefs built of coral fragments, which 
themselves are made of carbonate of lime that the 
coral animals have extracted from the ocean water. 
This in turn is dependent on the land for its supply 
of lime, which is obtained by the action of water in the 
rocks. ; 
Of this origin are the majority of the limestone beds 
of the world; and even now great strata of these rocks 
are being thus formed in the sea. We may have a 
limestone composed of fragments of shells, such as 
the coquina (Fig. 30) of the Florida coast; or it may 
be a coral rock fashioned almost entirely of pieces of 
coral; or it may be made of microscopic calcareous 
sea-shells, floating for a time at the ocean surface, and 
