130 INTRODUCTION. 
The winter of 1842, which was very severe, produced a 
similar effect on the naked rnollusks of the coast of 
Massachusetts. The species which usually are very 
abundant there were hardly observed during the suc- 
ceeding year. Oysters, spread by the fishermen on 
oyster banks for preservation, were destroyed at the 
same time in great numbers. Thus a series of long 
and uncommonly cold winters, or of cold and dry sum- 
mers, reduces their numbers to such a degree that 
scarcely an individual is seen where thousands were 
met with before. On the other hand a succession of 
warm and moist seasons increases their numbers incrcd- 
ibly. 
Inundations of Rivers. Tracts of land, but little 
raised above the level of high water, occur on the bor- 
ders of nearly all our large rivers. On the lower parts 
of the Ohio and Mississippi rivers especially, they con- 
stitute a wide, level, alluvial territory of great extent, 
comprising many hundred square miles. The periodical 
melting of ice and snow in the spring, and heavy rains 
in the mountains where they have their sources, cause 
such an increase in the volume of their waters, that 
they occasionally rise above their banks and overflow 
the low lands in their vicinity. These inundations are 
usually limited to a narrow region, and speedily sub- 
side ; but when, by a simultaneous operation of these 
causes over a wide extent of country, all the head 
waters pour their tribute at the same time into the 
main trunks, the mass of water becomes irresistible, 
