1882.] 
381 
[Davis. 
canic action, we can record few of these lakes. Thurston and 
Borax Lakes in the Coast Range of California are perhaps of this 
species ; more distinct are two water-holding craters on an island 
in Lake Mono. 1 
C. 15. Coral Island Lagoons. The peculiar growth of coral 
reefs, enclosing quiet lagoons, that remained so great a puzzle till 
explained by Darwin, is the combined result of a limitation in the 
depth at which reef-building corals can live, and of the slow sub- 
sidence of the volcanic island on which they colonized. The 
lagoons, when enclosed from the ocean by all but a small outlet, 
as happens on the smaller atolls, are essentially lakes, being 
bodies of quiet water held apart by barriers from the general cir- 
culation: their peculiarity is less in their basin than in the 
narrowness of their enclosing rims. Maraki, Henuake and Tai- 
ara may be given as examples of nearly complete enclosure of 
the lagoon waters. With further subsidence and corresponding 
upward growth, these circular islands decrease in diameter and 
the lagoon fills up with coral sand, as in Swain’s and Jarvis’s 
Islands. 2 A less usual change in the lagoons results from an 
elevation of the island, so as to give it drainage downward to 
the ocean, and allow the salt waters to be replaced by fresh 
rainfall : this is presumably the explanation of the lakes on 
Otdia, one of the Marshall group, 2 and on Washington Island 3 : 
little oases of fresh water in an ocean of salt. 
Having thus subdivided lakes according to their mode of form- 
ation, it remains to study their distribution and actual occurrence. 
This may be made the subject of a future paper. 
Mr. F. W. Putnam exhibited a number of aboriginal copper, 
bronze and silver objects from North and South America, and 
commented on the use of these metals by prehistoric tribes. 
General Meeting. February 1, 1882. 
Vice-President, Mr. F. W. Putnam in the Chair. Thirty- 
three persons present. 
1 Whitney, Geol. Cal. I, 97, 453. 
2 Dana, Corals and Coral Islands, 167, 283. 
3 Smithsonian Report, 1877, 20. 
