ORGANIC AGENCIES. 109 



scale on a scale commensurate with the formation of 

 continents and ocean-bottoms. The phenomena of coral 

 reefs show a down-sinking of the mid-Pacific bottom of 

 several thousand feet, and over an area, of many million 

 square miles. This has been going on through later geo- 

 logical times, and is probably still progressing. Now, so 

 wide-spread a downward movement must have its correla- 

 tive in an upward movement somewhere else. It seems 

 probable that we find it in the upheaval of the western 

 half of the American Continent, both North and South. 

 It is well known that during the whole later Tertiary, even 

 to the present time, the western part of North America, 

 especially the plateau region, has been slowly rising, the 

 extreme rise being nearly 20,000 feet. As it rose, the 

 general erosion became greater and the canons cut deeper 

 and deeper. So that the down-sinking of the Pacific bot- 

 tom, the upheaval of the plateau region, and the cutting 

 of the wonderful canons of that region, are probably all 

 connected with each other. 



REEFS AND KEYS OF FLORIDA. 



\The reefs of Florida deserve separate and special treat- 

 ment, not only because they are on our own coast, but also 

 because they are in some important respects entirely pecu- 

 liar : 1. In the Pacific, barrier-reefs are always the result 

 and the sign of subsidence. In Florida, on the contrary, 

 we have barrier-reefs where there has been no subsidence. 

 2. In the Pacific, corals do not add to the previously ex- 

 isting land-surface ; on the contrary, they only recover a 

 small fraction of a lost land-surf ace^ But in Florida there 

 has been apparently no loss, but a constant growth of land- 

 surface under the action of corals, assisted by waves and 

 other agents, as we shall presently explain. Attention 

 has not been hitherto sufficiently drawn to the entire 

 uniqueness of these reefs. 



