Limestones. 
227 
into three classes, accordingly as they are — (1) organi- 
cally, (2) chemically, or (3) mechanically formed. In 
a certain sense our Silurian limestones belong to all 
three divisions. They are of organic origin inas- 
much as every particle at one time or other belonged 
to a living organism — shell, coral, or crinoid. Much 
of the carbonate of lime in compact limestones has 
been chemically precipitated there, and some of the 
original carbonate has been dissolved out. In this 
sense limestones are partly of chemical origin. In 
the building up of coral reefs, their origin is partly 
mechanical , the action of the waves breaking coral 
from the outer edge of the reef, heaping up the 
fragments, and tilling up the interstices with broken 
material. 
In looking for fossils in ancient limestones, it is 
well to note that the corals and shells are best 
seen on weathered or on polished surfaces. It will be 
observed, too, that many limestones seem devoid of 
fossils over considerable areas, particularly where the 
limestone has been altered to a crystalline structure. 
Carbonate of lime is such a soluble material that lime- 
stone rocks may become completely altered in course 
of ages, merely by percolating waters dissolving and 
re-precipitating it. 
A little observation will not fail to make one note 
that some limestones are for the most part made up of 
shells. In this instance the material making the stone 
once formed an outer covering or defensive shell foi 
living creatures that were inhabitants of the ocean. 
