SEDIMENTARY AND METAMORPHIC ROCKS 87 
Every stream carries some mineral matter in solution. 
The hardness of water is a proof of this, for it is 
the dissolved mineral which makes the water hard. 
In some places, as for instance in the Dead Sea and 
the Great Salt Lake, streams enter basins from which 
there is no outlet. The pure water evaporates, leaving 
the chemical impurities behind; and, in course of 
Fic. 36. 
Caleareous tufa. 
time, the small contributions of the streams, render 
these waters more impure than the ocean. Since salt 
is one of the most common of the substances carried in 
solution, it is the most prominent impurity in these 
interior seas. In course of time, the water may be- 
come so salt that some must be deposited as rock salt. 
Beds of this precipitated mineral are sometimes several 
