306 NEW ZEALAND NATURE-STUDY BOOK 
by the water in its passage through the soil, so that when 
it reaches the roof of the cave, it contains Carbonate of 
Lime in solution. As each drop gathers on the roof and 
begins to evaporate and lose Carbon-dioxide, .the excess of 
Carbonate of Lime which it can no longer retain is de- 
posited round its edges as a ring. Drop succeeding drop, 
the ring gradually grows into a long hanging tube, called 
a stalactite—usually having a small aperture down the 
centre—which, by degrees becoming longer and _ thicker, 
may reach the floor, and develop into a massive pillar. 
The deposits on the floors of caves formed by water 
carrying Carbonate of Lime in solution, and depositing it 
under similar conditions, are known as stalagmites. 
Origin. Although included under the heading of 
sedimentary or aqueous rocks, Limestone is a rock of 
organic origin—that is, it has been formed largely, in some 
cases entirely, from the remains of minute organisms which 
have gradually accumulated on the seafloor. Many 
marine and fresh-water animals (shell-fish and other organ- 
isms) possess a covering or skeleton composed mainly of Car- 
bonate of Lime, and coral reefs consist of the same material. 
Deposits of organisms, from various sources, took place in 
ages past on the sea bottom—subsequent elevation raised. 
the deposits, converting them into dry land. Similar de- 
posits are now taking place in many of the seas—a study 
of which has thrown much light on the formation and 
growth of limestone. 
The rock may also be formed by a chemical deposit from 
calcareous springs—such deposits, however, are much less 
common and less extensive than those of organic origin. 
Originally formed deposits frequently lose their fragmental, 
and acquire instead a crystalline structure. 
Note.—The organisms which compose most varieties of Lime- 
stone live in deep water—hence Limestone has been formed 
further from land than either Sandstone or Shale—i.e., in clear 
water. 
