THEORIES OF BARRIER AXD CIRCULAR REEES. 155 



a foot, than during the summer. The falling of the water annually 

 clips this grove at the same level. !N"ow all the prongs at this level 

 were dead for about three inches. Evidently, therefore, this is the an- 

 nual growth of madrepore-prongs.* But in branching corals the rate 

 of point-growth is very different from the rate of ground-rising. If all 

 the points of a madrepore be cut off three inches, then ground into 

 powder, and the powder strewed evenly over the ground shaded by the 

 coral-tree, the elevation thus produced would approximately represent 

 the annual rate of reef ground-rising for this species. A quarter of an 

 inch would probably be a full estimate. This would make two feet for 

 a century. One foot to two feet per century is, therefore, probably 

 about the rate at which coral ground rises. As already stated, the rate 

 of subsidence may be less, but can not be greater, than this. 



Time involved, — At this rate 10,000 feet of vertical subsidence would 

 require 500,000 to 1,000,000 years. How much of this belongs to the 

 present geological epoch it is impossible to say. Dead corals, identical 

 with those still living on the reefs, have been brought up from a depth 

 of 250 feet, but, as this is only 150 feet below the limit of coral-growth, 

 it would require only 75 to 150 centuries. The process probably com- 

 menced in previous geological epochs, and has continued to the present 

 time. This is, therefore, an admirable example of geological agencies 

 still at work. 



Geological Application. — The facts brought out in the preceding 

 pages are of great importance in geology. 



1. "We have here the most magnificent example of subsidence still 

 in progress. The subsiding area has not been accurately defined, but 

 it probably covers nearly the whole of the intertropical Pacific. Ac- 

 cording to Dana, estimated by the atolls alone, it is 6,000 miles long 

 and 2,000 miles wide ; but if we take into account also barriers, which 

 are equally certain evidences of subsidence, it extends east and west 

 from the extreme of the Paumotu group on the one side to the Pelews 

 on the other, and north and south from the Hawaiian group to the Fee- 

 jees, making an area of not less than 20,000,000 square miles. Kow, it 

 is evident that there must have been, as a correlative of this extensive 

 and permanent downward movement, an equally extensive permanent 

 elevation of the earth's crust somewhere else. Dana thinks its correla- 

 tive is found in the extensive elevations of the Glacial epoch, and there- 

 fore that the whole work was accomplished since the Tertiary. But it 

 is more probable that its correlative is found in the gradual bodily 

 upheaval of the whole western side of the continent, especially in the 

 Rocky Mountain region, which commenced after the Cretaceous. 



* See full account of these observations in American Journal of Science and Arts, vol. 

 x, p. 34. 



