BARRIER REEKS, 293 



fringing reefs, and let this island with its reef, represented by the 

 unbroken lines in the wood cut, slowlv subside. As the island sinks 

 down, the reel continually grows upward; as the island subsides 

 the space between the inner edge of the reef and the beach be- 

 comes proportionally broader. A section of the reef and island 

 in this state is represented by the dotted lines, A A, being the 

 outer edges of the reef; C C, the lagoon; B B, the shores of the 

 encircled island. This section is a real one (on the scale of .388 

 of an inch to the mile), through Bolabola in the Pacific. We 

 can now see why the barrier reefs are so far from the shores which 

 they front. Supposing the island to still subside, the corals mean- 

 time growing vigorously upward, the last traces of land will finally 

 disappear, and a perfect atoll be formed. We thus perceive why 

 atolls so much resemble the harrier reefs in general size, form, 

 and manner in which they are grouped together, for they are but 

 the rude outlines of the sunken islands over which they stand. 

 In proof of the foregoing simple and not at all improbable cause 

 for the formation of barrier reefs, and atolls, Mr. Darwin gives 

 some examples of actual subsidence now in progress, and also 

 presents some evidence of the recent elevation of those islands 

 and coasts which have fringing reefs. The sinking of the islands, 

 or coast, for the formation of barrier reefs, or atolls, must neces- 

 sarily have been very slow, and undoubtedly large archipelagos 

 and lofty islands once existed, where now only rings of coral rock 

 scarce break the open expanse of the sea; thus the only record 

 left to us of the existence of vast tracts of land are the wonderful 

 memorials of these busy architects ; in each barrier reef we see 

 evidence of land subsided, and in each atoll a monument of an 

 island lost. Busy from the first ages of the world, when the 

 primeval seas had but a few groups of living beings, of the low- 

 est order of organization, the coral polype has toiled from day to 

 day, and year to year, and is toiling now. What mighty changes 

 have p;tssed over our giobe since that remote period in which the 

 Geologist is first enabled to trace the existence of living beings 

 upon the earth. How many tens of thousands of times the earth 

 has revolved around the sun, and how many hugo mountain 

 chains of granite have been disintegrated, and their scattered frag- 



