154 ORGANIC AGENCIES. 



estimated by the distance of barriers from their high islands, or by 

 soundings off atolls, to ascertain the height of these coral mounds, 



i .-■ - ..rr 



Fig. 126. 



or by the average height of the high islands of the Pacific. 1. The 

 average slope of the high islands of the Pacific is about 8°. Xow, as- 

 suming this slope (Fig. 126), a barrier, d, at the distance of five miles 

 would be 3,700 feet thick, and would represent a subsidence nearly to 

 that extent (Kad. : tan. 8° : : a d : d b)\ a distance of ten miles would 

 represent a vertical subsidence of 7,400 feet. Many barriers are at much 

 greater distance. 2. Off Keeling atoll 6,600 feet, a line of 7,200 feet 

 found no bottom (Darwin). Near other atolls a depth of 3,000 feet has 

 been found (Dana). 3. The average height of the high islands of the 

 Pacific can not be less than 9,000 feet (Dana) ; some of them reach 

 nearly 14,000 feet. It is very improbable that, among the hundreds of 

 atolls known, not one of their high islands should have reached the 

 average elevation of 9,000 feet. Yet these have entirely disappeared, 

 and not only so, but the small atolls and lagoonless islands, and more 

 especially the blank area, would seem to indicate that they have disap- 

 peared to great depths. For these reasons, it is almost certain that the 

 extreme subsidence has been at least 9,000 feet. We will take 10,000 

 feet as the most probable extreme subsidence. 



Rate of Subsidence. — The rate of subsidence may have been to any 

 degree less, but can not have been greater, than the rate of coral ground- 

 rising ; for otherwise the corals would have been carried below their 

 depth and drowned. It is difficult to estimate the rate of coral ground- 

 rising, but the only basis of such estimate is the rate of coral -growth. 

 Of the observations on this point we select two, one of them on the 

 head-coral (meandrina), the other on the staghorn-coral (madrepore) : 



1. On the walls of the fort at the Tortugas, Florida, meandrina com- 

 menced to grow, and in fourteen years the crust had become only one 

 inch thick. Agassiz takes one inch in eight years as a probable rate 

 under favorable circumstances. This would be one foot in a century. 

 As this is a head-coral, the coral-growth may be taken as the measure 

 of the reef ground-rising. 



2. In examining the reefs about the Tortugas in the winter of 1851, 

 an extensive grove of madrepore was found in the comparatively still 

 .water on the inside of the outer reef, in which the thick-set prongs had 



grown, year after year, to the same level, and were successively killed. 

 The mean level of the water here is lower during the winter, by about 



