26 LAKES OF NORTH AMERICA. 



Bassins due to org-anic ag-encies. — The study of coral reefs has 

 shown that bodies of sea water are sometimes cut off from the ocean, 

 although rarely completely separated, by the growth of reefs of 

 living coral adjacent to coasts, or as atolls about isolated islands and 

 " banks." Lakes of this nature perhaps occur at the south end of Florida, 

 and on the West India islands, but no well defined instances have been 

 described. 



The formation of peat in temperate latitudes rffords another illustra- 

 tion of the manner in which organic agencies lead to the formation of 

 lakes. The growth of the moss known as Sphagnum^ from which peat is 

 largely formed, may obstruct sluggish drainage ; and its unequal growth 

 in swampy areas leads to the formation of mounds with depressions in 

 their summits. The best known illustration of this type is Drummond 

 lake, in Virginia, but many smaller examples occur in other swampy 

 areas. It has been suggested that the basins in peat swamps may have 

 originated by the burning of the bogs during times of -excessive drouth. 

 That this might happen is evident, but no authentic case of such an 

 occurrence is known to the writer. 



On the vast tundras skirting the Arctic ocean in both the Old and 

 the New World, there are vast numbers of ponds and lakes held in 

 depressions in the frozen bogs, and surrounded by banks of moss and 

 other vegetation. These water-bodies have probably originated in various 

 ways, but in some instances their birth may be traced to the luxuriant 

 growth of vegetation in spring and early summer about the borders of 

 lingering snow banks. When the vegetation of the tundras awakens 

 after its long winter sleep, its growth is surprisingly rapid, and the snow 

 drifts that last longest are surrounded with luxuriant mosses and brilliant 

 flowers. When such accumulations of snow finally melt, the vegetation 

 on the areas they occupied is less in amount than on the surrounding 

 surfaces. The tundra increases in depth by the partial decay and 

 freezing of the, lower portion of the vegetation forming its surface, and 

 the greatest thickness of frozen soil occurs where the vegetation is most 

 luxuriant. For these reasons, the places where snow banks form year 

 after year, become depressed in reference to the general surface, and give 

 origin to lakes. 



In sub-Arctic regions, as on the Aleutian islands, mosses and 

 herbaceous vegetation grow luxuriantly, and among the hills sometimes 

 obstructs the drainage by reason of the formation of a deep peaty soil by 

 its partial decay. 



