158 



ON THE FORMATION OF COBAL-BEEFS. 



[Feb. 21, 



all were obtained, either alive or dead, only great quantities 

 of these low-brauching Polyzoa. When we left the Australian 

 coast for the Arafura Sea these general characters were absent. 



Another peculiarity of this region was the great turbidity of the 

 water near the coast and the large amount of slimy mud deposited 

 on the flats, which as f ringing-reefs were everywhere present, 

 their seaward edge being marked by isolated, blackened, and 

 elevated masses of coral-rock. 



With such a considerable rise and fall of tide, 20 feet at springs, 

 when walking on the reefs one was struck by seeing the large 

 number of corals which were apparently able to stand 

 a prolonged exposure to the blazing sun of this nearly 

 equatorial latitude. Along with these tides there were strong 

 and powerful currents, the average temperature of tlie water being 

 80°. 



[- 



Eeef on Trougbton Island, N.W. Australia, 

 ■direction of current ; soundings in fathoms. Tide-rise 20 feet.] 



On the north end of Troughton Island, Lit. 13° 40' S., long. 

 126° 10' E., the condition was as follows at nearly low-water 

 springs : — The beach at high-water level was sand and shell, then 



