252 Mr. John Murray [March 16, 



this universal agency are most likely to be at fault. We know 

 something about the rate of solution, probably more than we do 

 about the rate of growth and secretion of carbonate of lime by the 

 coral polyps. It has been shown that the rate of solution varies 

 with temperature, with pressure, and with the amount of carbonic 

 acid present in the water. It is on the i)lay of these two opposing 

 forces — the one vital and the other chemical — and their varying 

 activity in different regions and under different circumstances, that 

 we rely for the explanation of many oceanographical phenomena, 

 esjiecially many of those connected with oceanic deposits and coral 

 reefs. In some regions there may be more growth, secretion and 

 deposition of shell and coral materials than solution by sea water, 

 and then there results the formation of coral reefs and vast calcareous 

 deposits at the bottom of the ocean. There may be an almost exact 

 balance between these processes. And again, there may be more 

 solution than secretion, as, for instance, in the red clay areas, which 

 occupy the deepest parts of the ocean, and in some coral reef lagoons. 



"What is the nature of the foundations of these coral islands, 

 surrounded as they sometimes are by an ocean miles in depth ? Why 

 have some elongated reefs no lagoons? Why have most of the 

 lagoons of the smaller atolls been filled up ? Why is the circle of 

 land or reef in the perfect atolls only, at most, a few hundred yards 

 in diameter ? What is the origin of the lagoon ? What relation 

 exists between the depth of the lagoon, its area, and the depth of the 

 water beyond the outer reef? How has the dry land of these islands 

 been formed, provided with a soil, a fauna and a flora ? These appear 

 to be the chief questions that demand an answer from any theory of 

 coral island formation. 



These coral formations are essentially structures belonging to 

 the gre'at oceans and ocean basins. They are dots of land within 

 the oceanic areas that might be compared or contrasted with tlie 

 small salt lakes which are scattered over the surface of the continen- 

 tal lands. A rapid survey of some of the more general phenomena of 

 the great oceans may, then, lead to a better appreciation of the 

 problems connected with coral reefs. 



The great ocean basins occupy over two-thirds of the earth's 

 surface, and have a mean depth of over two miles. The central por- 

 tions of these basins, called the abysmal regions, occupy about one- 

 half of the earth's surface, and have a mean depression below the 

 general level of the continents of over three miles. The abysmal 

 regions are vast undulating plains, sometimes rising to less than two 

 miles from the surface of the sea, and again sinking to four and five 

 miles beneath it. Volcanic cones rise singly or in clusters from 

 these great submerged plains. When they shoot above the level of 

 the sea they form single islands, like Ascension and St. Paul's Kocks, 

 or groups, like the Azores, the Sandwich, the Fiji, and the Society 

 Islands. As might have been expected, there are many more of these 

 cones hidden beneath the waves than rise above them. When the 



