COLOR INFLUENCED BY LOCAL CONDITIONS 171 



erals which are deposited near the rivers. The evidence 

 is only circumstantial, hut is very much in favor of the 

 mineral origin of the colors of pearls. From the infor- 

 mation which has been gathered it seems as though the 

 presence of iron is conducive to pink, or various shades 

 of red. Green seems to come from copper, while the 

 blue or lavender may be derived from lead or zinc or 

 from a combination of these minerals. Possibly the mus- 

 sels which live in a certain stream where a certain kind 

 of mineral is abundant gradually become accustomed to it 

 and feed at times when the water contains a large per 

 cent, of the mineral. 



In this manner the dissolved mineral may become as- 

 similated like the carbonate of lime and used with it in 

 the construction of the shells and pearls. Farther down 

 the streams the shells grow whiter, which seems to indi- 

 cate that the minerals which were in the water near the 

 source of the river have been gradually reduced by the 

 addition of water from other streams and to some ex- 

 tent by settling. As the river becomes larger the erosion 

 is gradually reduced. 



The size of the shells seems to depend upon the 

 amount of carbonate of lime in the water. This is illus- 

 trated by the sizes of the shells found in different parts 

 of the Illinois River. 



The river north of Peoria is enlarged by the waters 

 from smaller rivers, which carry a large quantity of car- 

 bonate of lime into it, and the shells in that district are 

 very large. The southern portion of the river is much 



