OCR Text |
Show 158 ON THE FORMATION OF CORAL-REEFS. [Feb. 21, all were obtained, either alive or dead, only great quantities of these low-branching 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 fringing-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 the Avater being 80°. E; f>W0§ Reef on Troughtou Island, N . W . Australia. [-«- direction of current; soundings in fathoms. Tide-rise 20 feet.] On the north end of Troughton Island, lat. 13° 40' S., long. 126° 10' E., the condition was as follows at nearly low-water springs :-The beach at high-water level Avas sand and shell, then |