130 INTRODUCTION. 



The -winter of 1842, which was very severe, produced a 

 similar effect on the naked moUusks 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 incred- 

 ibly. 



Inundations of Rivers. Tracts of land, but httle 

 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, 

 comprismg 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, 



