96 GEOLOGY: T. IV. VAUGHAN 
temperature so low as this. Wherever the depth of water is great 
enough to lower the bottom temperature below 18.15°C., more probably 
about 21°C., reef corals will not live. This temperature appears to be 
attained around the Hawaiian Islands within a depth of 183 meters. 
According to Agassiz's Three Cruises of the Blake the bottom tempera- 
ture in the Gulf of Mexico and Caribbean Sea is usually too low for the 
growth of reef corals at a depth of 183 meters, and in places it is too low 
at a depth of 87 meters. Although the possibility of control of the lower 
bathymetric limit of reef-building corals by decrease in temperature 
with increasing depth has not been adequately investigated, it appears 
safe to say that reef corals are usually, if not always, confined by tem- 
perature to water less than 180 meters deep. 
The four possible factors which tend to limit the downward extent 
of reef forming corals are as follows: (1) effect of sediment, (2) de- 
crease in supply of small animal plankton, (3) decrease in intensity of 
light, (4) lowering of the temperature. 
The relations of corals to salinity will now be considered. The 
average salinity of the Tortugas water according to Dole is 36.01%. 
Seventeen species of the Tortugas corals were kept in a large tank of 
water with a salinity of 18.28% for 24 hours. All were damaged or 
killed except Maeandra areolata, Siderastrea radians, and Porites as- 
treoides; but no specimen of 16 species showed any evidence of harm after 
remaining 48 hours in water of a salinity of 27.87%. Apparently corals 
would not be hurt if the salinity of the ocean were reduced to about 
80% of its present salinity. Although I did not experiment with con- 
centrated sea-water, the studies made by Goldforb and others on the 
effect of concentrated and diluted sea-water on regeneration in hydroids 
and in the Cassiopea are here pertinent. The combined results of the 
experiments are in accord with the deductions made by oceanographers 
and geologists from other data, viz., the ocean is becoming more salt, 
and it appears that marine organisms are now living in an environment 
which is considerably below the optimum condition for their existence. 
In order to ascertain the amount of atmospheric exposure corals would 
endure, experiments were made on 16 species, any of which will endure 
exposure on a glass plate in the shade for half an hour without apparent 
damage; nearly all will stand an hour without harm, while some will 
stand 4 hours' exposure under the conditions stated. Favia fraguni, 
Porites clavaria, and Porites astreoides have the greatest capacity for 
withstanding atmospheric exposure, while that of Maeandra areolata 
and Siderastrea radians is almost as great. A number of species with- 
stood exposure on a glass plate in the sun for \\ hours, the specimens 
