STRUCTURE OF CORAL REEFS. 121 



These tidal currents often have great strength, and are much 

 modified and increased in force at certain places, or diminished 

 in others, by the position of the reef with reference to the 

 land. Sweeping on, they carry off the coral debris from some 

 regions to others distant ; and again they bear along and dis- 

 tribute only the shore detritus. It is thus seen that the same 

 region may differ widely in its adjacent parts, and seemingly 

 afford evidence in one place that there is no coral near, and in 

 another no high land, although either is within a few rods, or 

 even close alongside. 



The extent of the land in proportion to the reef will have 

 an obvious effect upon the character of the channel or lagoon 

 depositions. When the island stands, like one of Bacon's Isles 

 in the Feejees, as a mere point of rock in a wide sea in- 

 closed by a distant barrier, the streams of the land are small 

 and their detritus quite limited in amount. In such a case, 

 the reef, and the growing patches scattered over the lagoon, 

 are the sources of nearly all the material that is accumulated 

 upon the bottom. 



The bottom between the inner reefs within the great Aus- 

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

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

 bled 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 neighbouring 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. 



