STRUCTURE OF CORAL REEFS. 107 



The great Australian barrier forms a broken line, twelve 

 hundred and fifty miles in length, lying off the coast from the 

 Northern Cape to the tropical circle. 



The seas outside of the lines of coral reef are often un- 

 fathomable within a short distance of the line of breakers. 



In the further description of reef-grounds, or reef-formations, 

 there are several distinct subjects for consideration, as is 

 obvious from the preceding remarks. These are — 



1. Outer reefs ^ or reefs formed from the growth of corals 

 exposed to the open seas. Of this character are all proper 

 barrier reefs, and such fringing reefs as are unprotected by a 

 barrier. 



2. Inner reefs, or reefs formed in quiet water between a 

 barrier and the shores of an island. 



3. Channels, or seas within barriers, wliich may receive 

 detritus either from the reefs, or from the shores, or from both 

 of these sources combined. 



4. Beaches and beach formations, produced by coral accumu- 

 lations on the shores through the action of the sea and winds. 



The outer and inner reefs, channels, and beaches, act each 

 their part in producing the coral formations in progress about 

 i.-.lr.nds. 



II. OUTER RERFS. 



The barrier and other outer reefs are always submerged at 

 high tide, except where elevated at surface by accumulations 

 of beach sands. The level is generally that of about one- 

 third tide. The coral rock is built up by the agencies at work 

 to this level, and hence the existence of the broad platform- 

 like top of the barrier. The surface is however not even, for 

 there are many pools of water over it, even at the lowest tides, 

 especially toward its outer limits, where corals of various kinds 

 are growing luxuriantly, with fit associates of shells, star-fishes, 

 echini, holothurias with their large flower-bearing heads, 

 sponges, corallines and sea-weeds, making scenes of rare 

 beauty. The growing corals are, however, most abundant 



