1 68 



MARINE DEPOSITS 



Florida reefs, and in storms the water is often discoloured and 

 turbid for miles around the reef. The sea-water dissolves and 

 redeposits CaC0 3 , cementing the fragments into a firm rock, 

 which, especially after exposure to the air, may become very hard. 

 By these processes several varieties of rock are formed, corre- 

 sponding, in all but the material, to the ordinary marine deposits. 

 In one form the standing and unbroken colonies are filled up with 

 calcareous debris and enclosed in solid masses. Coral conglomer- 



mw^. 





■sir. f -.-£*fl^.. 



- 



w£L'_ t 



FIG. 67. — Corals on the Great Barrier Reef of Australia, mostly different from 

 those in Fig. 66. (Savile Kent.) 



ate or breccia is a cemented mass of coral pebbles or angular pieces, 

 or is made up of fragments of an older coral rock. Reef rock is 

 the dense and solid mass formed by the cementing of the finer 

 debris which accumulates in quiet water. It is important to notice 

 that even under the microscope reef rock frequently shows no 

 trace of organic structure, and is a definite proof that the absence 

 of such structure is not a sufficient reason for denying the organic 

 origin of a rock. The interior of growing masses which are still 

 alive on the outside, and have never been broken up, may be so 

 crystallized by the action of the sea-water that the organic struct- 

 ure is obscured or destroyed. On the beach is formed a curious 



