238 THE AMERICAN MIDLAND NATURALIST 



compound. As the result of these interactions the ground beneath 

 humus deposits is usually found bleached. Clays, originally red 

 or yellow, may become black, green or blue from the organic matter 

 contained and from the effects of this process. When clay is burned, 

 the organic matter and ferrous carbonate contained is oxidized, 

 and red brick is formed for whose red color the iron oxide is re- 

 sponsible. Now analyses of the shells of 4 common species of 

 Najades furnished through the courtesy of the Commissioner of 

 Fisheries, Washington, D. C, show that the differences in composi- 

 tion between them is greatest and most marked in the content of 

 oi-ganic matter, iron, alumina and phosphoric acid. The first three 

 of these are important constituents of soil, and are known to be 

 responsible for the colors of many minerals. The greater rapidity 

 of current, rapids, etc., of the headwaters of streams makes for their 

 greater oxygenation, and this available oxygen may so react with 

 these minerals of the shell as to produce the deeper colors character- 

 istic of the headwaters in a fashion corresponding to the processes 

 described. Further down stream, silt and slow moving waters as 

 well as other conditions may tend to inhibit the oxidative process, 

 and, similarly there may follow a greater inclusion of organic matter 

 in the shell, possibly from the silt itself, resulting in other than the 

 red or yellowish colors allied to iron oxide. A relatively similar 

 situation is found in L. Erie, which has less silt bat lighter colored 

 shells. The lighter colors of L. Erie shells may be closest related to 

 the degree of alkalinity of the water. As the natural conditions 

 of the Upper Ohio Drainage are also largely similar to those of the 

 streams draining into L- Erie, a similar explanation may be assumed 

 for their colors. Another alternative hardly consistent with the 

 above facts is that the concentration of humic acid toward the 

 mouth of the stream becomes sufficient to bleach or lighten the 

 colors whose basis is iron. The improbability of this latter becomes 

 clearer when it be remembered that lime also increases in amount 

 going down stream. A more reasonable explanation is that CO2 

 whether of the soil water or released from combination by the 

 interaction of humic acid and lime, attacks the ferric or ferrous 

 oxide, already present in the shells and produces ferrous carbonate, 

 which colorless compound may be responsible for the lightening 

 of hues observed. Such an explanation at least embraces most of 

 the physical and chemical conditions known, and is certainly applic- 

 able to the many curious facts concerning the nacreous color known 



f 



