170 MARINE DEPOSITS 



calcareous sand. The latter may be further piled up by the winds 

 into dunes and solidified by the cementing action of percolating 

 rain-water. According to circumstances, the new platform may be 

 an extension of the shore or an island like the Florida Keys. 



Coral reefs are classed according to their relation to the shore, and 

 are of three kinds, (i) Fringing reefs. are those attached directly 

 to the land, though the exposed part may be at some distance out 

 from the shore and separated from it by a shallow channel with 

 coral bottom. The width of a fringing reef is determined by the 

 slope of the sea-bottom, being narrower on a steep grade, broader 

 on a gentle one. (2) Barrier reefs are farther out from shore, to 

 which the reef is parallel in a general way, and separated by a 

 broad and often quite deep channel. The distinction between the 

 two kinds of reefs is not very sharply drawn, for the same reef may 

 be fringing in parts of its course and a- barrier in others. Even 

 at the present time barrier reefs are sometimes constructed on an 

 enormous scale. A great barrier reef runs parallel to nearly the 

 whole north shore of Cuba, while the barrier reef of Australia, the 

 largest known, extends, with some breaks, for over 1200 miles 

 along the northeast coast of Australia, from which it is distant 

 20 to 80 miles ; its breadth "varies from 10 to 90 miles, though but 

 little of this width is exposed above water ; its sea-face is in some 

 places more than 1800 feet high {i.e. above the sea-bottom, not the 

 surface). (3) Atolls are coral islands of irregularly circular shape, 

 which usually enclose a central lagoon and frequently, as in the 

 Pacific, rise from the profoundest depths. The way in which such 

 islands have been built up is still a subject of much controversy, and 

 limitations of space forbid its discussion here. 



Dolomitization. — A process has been observed in the closed 

 lagoons of certain atolls which is significant as throwing light upon 

 a very difficult problem, that of the formation of dolomite or 

 magnesian limestone (see p. 213). In the closed lagoon, shut off 

 entirely from the sea, the isolated body of sea-water becomes con- 

 siderably concentrated by evaporation. All sea-water contains 

 chloride of magnesium (MgCl), and this percolating into the coral 

 rock, by double decomposition with CaC0 3 , forms MgC0 3 . The 



