CORAL REEFS. 17 



perature is 85° F., or two degrees higher than in the Atlantic. The mean temperature 

 for the 3'ear is, in the North Pacific 73.5 F.; in the South Pacific, 70" F. Where the 

 temperature of the surface is never below 70' F. during the year, that is within i5"-20° 

 of the equator, the reef corals abound both in species and individuals, as at the Fiji 

 Group, which is one of the most remarkable coral gardens of the ocean. The Hawaiian 

 Islands are near the northern limit of subtorrid warmth and only the hardier forms are 

 found (as Pontes and Poci/Iopora) and their growth is not so luxuriant: the beautiful 

 Madrepora of the southern groups is wholly wanting. This brief reference must 

 suffice to indicate the important facflor that temperature makes in the distribution 

 of reefs. Corals will not grow in niudd\- water, or when the percentage of salt falls 

 below a certain point, hence their absence opposite the discharge of rivers. In 

 depth the living corals (reef-building) do not extend beyond twenty-five fathoms 

 or 150 feet (Dana). 



The Hawaiian Islands are well j^rovided with fringing reefs but have no 

 barrier reefs, and these two forms are thus distinguished: the former is a fringe or ex- 

 tension around or on certain coasts of a high island, presenting a tolerably flat surface 

 at low tide, interrupted by wells and channels ; the latter is detached from the shore 

 by a channel of greater or less width, and may form a wing encircling the island, or it 

 may extend along a coast as the Great Barrier Reef of the east coast of Australia 

 which extends parallel with that coast some 1250 miles. What is the explanation of 

 these detached reefs ? It is not so difficult to understand the growth from a shore 

 as the polyp grows, comes too near the surface, is exposed too long at low tide, dies 

 and its successors have to push seaward. On most fringing reefs the dead far out- 

 numbers the living coral. If coral, probably from a deficiency of light, cannot grow 

 at a depth below twenty-five fathoms, how could a detached mass start from the bottom 

 of an ocean which in the immediate vicinity of most coral islands presents a much 

 greater depth ? Charles Darwin explained this in a very simple way and his conclu- 

 sions, with all their consequences, were accepted as satisfactory for many years. It is 

 well known that changes of level take place in "solid" land. On the Hawaiian island 

 Oahu the ancient coral reef is now from two to three fathoms above the level at which 

 it was formed not many ages ago, and other regions have as evidently subsided. In 

 this subsidence Mr. Darwin finds the key to the formation of barrier reefs. Granted 

 the subsidence this theory capitally explains all the phenomena of reef formation. 

 Agassiz, Dr. Murray and Professor Alexander Agas.siz (feeling that the subsidence 

 theory was not proven for all localities) base their explanation of the barrier reef 

 on the growth of the coral on the rim of a volcanic crater at a suitable depth. There 

 is this difficulty that some of the atolls in the Indian Ocean would presuppose a crater 

 thirty miles in diameter, a size which has no parallel on the earth's surface. Interest 

 has lately been excited in this question by the borings on the coral island of Funafuti, 



Memoirs B. P. B. Museum, Vol. I., No. 2.-2. 



[lOl] 



