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LETTER VI. 



THE MOLLUSCA CONSIDERED IN THEIR RELATIONS TO 

 INORGANICAL OR DEAD MATTER. 



IN the preceding letters, I have illustrated, at some length, 

 the relations in which the mollusca stand to man and other 

 animals, and you are satisfied that even in that view they 

 are worthy our attention. I now proceed to indicate more 

 briefly the results they operate on dead and inorganic 

 matter, their share of influence in the construction of the 

 glohe as it now exists, and in those changes which are gra- 

 dually altering its present aspects. 



In all countries, but especially in those where a warm or 

 tropical temperature clothes the surface with forests, which 

 extend unmeasured shades along the banks of their mighty 

 rivers, an inconceivable quantity of trees are annually floated 

 to the ocean during the rainy season. When, by any cause, 

 a portion of these has been stopped in its descent, the course 

 of the river is turned aside, and an island has been the con- 

 sequence ; and there can be little doubt, that in the lapse of 

 ages even the deep sea would labour under the load bays 

 would be filled, and the mouths of rivers and harbours ob- 

 structed for wood, when entirely submerged, is almost 

 indestructible under the mere influence of water. But the 

 Teredo, and one or two allied mollusks, have received their 

 commission : under their augers, worked by an instinct which 

 allows of no repose, and no misapplication of their powers, 

 the timber is drilled through, crumbled down, and removed 

 as fast as it is supplied. " The seaman," to adopt the rather 

 pompous language of a very excellent author, "as he beholds 

 the ruin before him, vents his spleen against the little tribes 

 that have produced it, and denounces them as the most mis- 

 chievous vermin in the ocean. But a tornado arises, the 

 strength of the whirlwind is abroad, the clouds pour down a 

 deluge over the mountains, and whole forests fall prostrate 

 before its fury. Down rolls the gathering wreck towards the 

 deep, and blocks up the mouth of that very creek the seaman 

 has entered, and where he now finds himself in a state of 

 captivity. How shall he extricate himself from his impri- 

 sonment ? an imprisonment as rigid as that of the Baltic in 



