1882 .] 
869 
[Davis. 
C. 7. Barrier Reef Basins. The lakes of the Everglades, in 
Southern Florida, owe their original enclosure from the ocean to 
the growth of coral reefs parallel to the present shore but farther 
inland. 1 The corals, aided by wave, wind and plant action, build 
a more or less complete barrier ; the lagoon behind it becomes 
shallowed on either side by silt from the land and coral-sand from 
the reef, until all its margin becomes swampy ; the water surface 
is raised by a narrowing of its outlet, and so a flow from the land 
is established, and finally its central part remains as a shallow 
freshwater lake, with low swampy shores. 
C. 8. River Bagoons. ( Ox-bows: Aigues-mortes.) Rivers of 
gentle slope running through a flat bottom-land of considerable 
breadth, are both curved and unsettled in their course ; if made 
straight artificially, they tend to relapse into their old crooked 
ways ; the first snag or sand-bar would divert the direct current 
and turn it against the bank, and so a curve would be formed that 
propagates its disturbance up stream as well as down. The size 
of such curves or meanders depends mostly on the volume and 
slope of the stream ; as the bends become more pronounced, their 
necks approach each other by the caving in of the concave banks, 
and at last, often by the aid of overflow during floods, a short cut 
is made which diverts the stream from its circuitous course into a 
shorter and consequently steeper channel. As soon as this change 
is accomplished, the entrance and exit of the great bend or ox- 
bow begin to silt up and it is shortly transformed into a marshy 
lagoon. An unexpected result is the discovery that these lagoons 
may be deeper than the adjoining channel; this is because the 
stream in such a case is continually building up its bed by silt 
deposits ; consequently the lagoon which represents an old chan- 
nel will have a lower bottom than that of the present river. 2 
The lower Mississippi has many crescent-shaped lagoons of 
this origin in all stages of separation from the river, and of 
extinction by silting up 3 ; some are noted in Iowa also 4 ; they 
1 L. Agassiz, Report on the Florida Reefs, Cambridge, Mus. Comp. Zool. Mem., vi, 
1880, 32, and Leconte, Elements of Geology, 149. 
2 Warren, Amer. Journ. Sci., xvi, 1878, 422. 
3 Humphreys and Abbott, Report on the Mississippi River, plate n. 
4 White, Geol. Iowa, i,70. 
PROCEEDINGS B. S. N. H. VOL. XXI. 
24 
JULY, 1882. 
