152 ORGANIC AGENCIES. 



which are perfect atolls, but, by sounding, the head of the drowned vol- 

 canic island is still detectable. The next step in the series is the perfect 

 atoll, then the small atoll, and, finally, the lagoonless coral island. These 

 last kinds show that the original island has gone down deeply. 3. By 

 grappling-hooks dead coral-trees have been broken off and brought up 

 from the ground where they once grew, now far below the limiting 

 depth of coral-growth. The evidence of subsidence in this case is of the 

 same kind and force as that derived from the submerged forest-ground 

 (page 136). The corals have been carried below their depth and drowned. 

 4. The remarkable distribution of the various kinds of reefs brought to 

 light by Dana is satisfactorily explained by this theory, and therefore is 

 an argument in its favor. In the middle of the atoll region of the Pa- 

 cific there is a blank area, 2,000 miles long and 1,000 or more miles 

 wide where there are no islands. Next about this is an area in which 

 small atolls predominate ; about this again the region of ordinary atolls ; 

 beyond this the region mostly of barriers, and finally of fringes. Now, 

 by this theory this distribution is thus explained : The sea-bottom in 

 the blank area has gone down so fast that the corals have not been able 

 to keep pace, and have therefore been drowned, and left no monu- 

 ment of their existence. In the next region the corals have been able 

 to keep within living distance of the surface, but the original islands 

 have not only disappeared, but gone down to great depths. In the 

 next the original high islands have disappeared, but not gone down so 

 deej) ; in the next they have sunk only to the middle. The fringing 

 reefs stand on the margin of the sinking area. Outside of this again 

 there is in some places even evidence of upheaval instead of subsidence. 

 Raised beaches in the form of fringing- reef rocks are found clinging 

 to the sides of high islands many feet above the present sea-level. 5. 

 In some places this subsidence seems to be still in progress. On cer- 

 tain coral islands sacred structures of stone made by the natives are 

 now standing in water, and the paths worn by the feet of devotees are 

 now passages for canoes (Dana). 



Murray's Theory. — Recently serious doubts have been cast on Dar- 

 win's subsidence theory, at least as a universal explanation of barriers 

 and atolls.* Mr. Murray, from his observations during the voyage of 

 the Challenger, believes that barriers and atolls may be explained with- 

 out subsidence of the sea-floor. An outline of his views may be thus 

 stated : 1. Submarine banks formed in any way, either (a) built up by 

 accumulating shells of successive generations of marine animals, until 

 within the reach of coral-growth ; or (b) by volcanic cinder cones cut 



* The author of this volume believes that he was the first who showed, in the case of 

 the Florida reefs, how barriers may be formed without subsidence. American Journal 

 of Science and Art, vol. xxiii, p. 46, January, 1857. 



