THE COLLECTION OF MINERALS 
A chemical feature of the greatest importance in the con- 
stitution of the silicates is the replacing power of the bases, 
whereby one or more basic molecules take the place in whole or 
in part of another in the chemical composition of a mineral. For 
instance, the sesquioxide of alumina (Al,O,) can be replaced by 
the sesquioxide of iron (Fe,O3), except in the feldspars; and, 
similarly, lime (CaO), magnesia (MgO) and protoxide of iron 
* 
CALCITE PSEUDOMORPH AFTER ARAGONITE, CIANCIANI, ITALY 
(FeO) are substituted for one another; or any of these can take 
the place of the oxides of potassium and sodium (K,O, Na,O). 
This replacing process comes prominently into view in Garnet, in 
which with the same theoretical formula for a very large group 
the composition of the different varieties is different through the 
effect of this law. Another, and simpler, instance is the series of 
compounds connecting Siderite (FeCO,) with Calcite (CaCO,). 
Among the silicates may be mentioned Feldspar (Desk Cases 
to and 11, Wall Case R), Pyroxene (Desk Case 11), Amphibole 
16 
