292 THE 



extending sometimes as far as a mile from the land, and always 

 to such a distance from the shore that the limiting depth of 20 or 

 30 fathoms is obtained, where the reef ceases. From the more 

 flourishing growth of the outermost corals, the fringing reefs are 

 usually highest at the outside, and the sediment washed inwards 

 upon the reef, generally produces in the course of time, a shallow 

 sandy channel* Such are the three great classes of coral reefs 

 which are found scattered throughout the vast oceans, and princi- 

 pally in the tropical regions, but it must by no means be supposed 

 that they are found indiscriminately united, on the contrary the 

 atolls and barrier-reefs are never found in proximity to the fring- 

 ing reefs. It has been remarked with surprise that while atolls 

 are the most cojnmon coral structures throughout some vast por- 

 tions of the ocean, such as the tropical Pacific and the Indian 

 Oceans, they are entirely wanting, or very nearly so, in the tropi- 

 cal Atlantic and West Indian Seas, where the corals themselves, 

 are exceedingly numerous. There is also another somewhat re- 

 markable fact, that no single active volcano occurs within several 

 hundred miles of a coral archipelago, or even a small group of 

 atolls ; and although most of the islands in the Pacific which are 

 encircled by barrier-reefs are of volcanic origin, having remains 

 of craters distinctly visible, yet not one of them is known to have 

 been in eruption since the growth of the corals. In explaining 

 by any theory the formation of coral reefs, we must consider all 

 the phenomena presented by the three great classes as enumera- 

 ted in the preceding description. To ourselves the explanation 

 proposed by Mr. Darwin in his volume upon * The structure and 

 distribution of Coral Reefs," is the most satisfactory, and maybe 

 briefly stated thus; islands, or a line of coast, being first skirted 

 with fringing reefs, become atolls by a continual but gradual subsi- 



LEVEL Or SE 

 dence of the land. Let us then take an island surrounded by 



