152 CORALS AND CORAL ISLANDS. 



tralian barrier, according to Jukes, as brought up by the 

 dredge from depths of fifteen to twenty fathoms, often resem- 

 bles the unconsolidated mass of a shelly or coralline limestone. 

 At other times it consisted very largely of the small disk-shaped 

 .foraminifers called Orbitolites, closely allied in form and na- 

 ture to the Nummulites of the Tertiary ; and they seemed 

 in some places to make up the whole sand of the beaches, both 

 of the coral islets and of the neighboring Australian shores. 



The facts show that the rock formed in such channels may 

 be of all the kinds that occur in reef regions — coral and shell 

 conglomerates, compact impalpable limestones, limestones full 

 of Orbitolites, or containing, as well, remains of other species of 

 the seas, and also rocks made of the clay, mud, sand or pebbles 

 of the mountains or high lands adjoining. 



VI. BEACH SAND-ROCK. 



Besides the ordinary coral rock, there are also beach for- 

 mations made of coral sands, worn shells, etc., thrown up by 

 the tides and waves. Their mode of formation is like that 

 of any sea-beach. The material is mostly like common sand in 

 fineness, but often much coarser. When the beach is fronted 

 by a distant barrier to shield it from the force of the waves, 

 the material is usually sand and small pebbles ; but if the reef 

 is narrow, so that the sea breaks over it with full force, it may 

 consist even of cobble stones, as on any other shore, and in- 

 clude also huge masses of coral rock. 



These deposits become cemented by being alternately mois- 

 tened and dried, through the action of the recurring tides and 

 the wash of the sea on the shores. The waters take up some 

 carbonate of lime, and this is deposited and hardens among the 

 particles on the evaporation of the moisture at the retreat of 

 the tides. In some places the grains are loosely coherent, and 



