ioo PEARLS AND PEARLING 



which contain the largest numbers of mussels are those 

 which contain pure water running over beds of lime- 

 stone rock in some places along their course. 



Rocks of various kinds and gravel and sand are good 

 signs of both shells and pearls. Limestone is very valu- 

 able in the formation of the shells, yet they grow well 

 in rivers where sandstone, slate, shale, marble or flint are 

 commonly found. As the water flows over the stones, 

 the process of erosion wears very fine particles away 

 from the rocks and this becomes mixed with the water, 

 making it calcareous, and is just what is needed for the 

 formation of shells and pearls. 



Some rivers have very little rock or gravel in them, 

 and their beds are muddy, yet if there are rocky bluffs 

 nearby, the smaller streams carry sufficient quantities of 

 lime water into the river to provide for the growth of 

 shells and so the muddy river also produces some shells 

 and pearls. 



Some varieties of mussels do not form beds, but are 

 distributed over a large area and scattered among other 

 shells. 



Other varieties live in colonies or beds. The beds are 

 of various sizes; some are rather small, while others 

 are a mile or more long. In some narrow rivers the beds 

 may cover the entire river bottom, while in the larger 

 streams they are often in very narrow beds. The posi- 

 tion of the beds depends upon a number of conditions 

 such as the nature of the river bottom, the strength of 

 the river current, the depth of water, the position of 

 islands and sand-bars, and other influences. Some beds 



