86 ELEMENTARY GEOLOGY 
Deposit in the Rocks. While chemically precipitated 
rocks are interesting, they are not very important; but 
chemical precipitation is of great consequence in the 
rocks. As it passes through them, water often takes 
a mineral in solution at one place, and deposits it in 
another, and this is one of the ways in which rocks are 
cemented (see pp. 52 and 275). In its passage through 
the earth, too, water is often producing change by a 
series of chemical reactions, the nature of which cannot 
be considered here. In some places entire beds of rock | 
are completely changed in kind by this chemical action» 
of water. For instance, certain limestones have been 
altered to magnesium limestone, or dolomite, while 
others have been changed to iron beds by the precipi- 
tation of siderite, or other salt of iron, from some 
solution of iron in water. Some valuable iron-ore 
deposits (such as those of the Lake Superior district) 
are of this origin. In a similar manner limestone 
beds have been changed to gypsum. 
Deposit by Evaporation. The last important way 
in which the chemical action of water is at work in 
making beds of rock, is by evaporation. In almost: 
any region, but particularly im an arid country like 
that of the Far West, springs, or even streams, 
evaporate and leave behind the load of chemically 
deposited material which they carried. Thus calcare- 
ous tufa is deposited (Fig. 36). 
