MINERALS OF BELT OF WEATHERING. 519 



are the olivines and similar minerals. The third group is the pyroxenes 

 and amphiboles. Fourth is the biotite-muscovite group. The fifth and 

 most difficultly decomposable important group is the feldspars. Still more 

 difficultly decomposable are some of the subordinate silicates. Of these 

 garnet, staurolite, tourmaline, andalusite, fibrolite, cyanite, and zircon are 

 probably the more important. The decomposition of these minerals is a 

 relatively slow process. The order given is that of relative decomposabilitv. 

 Garnet alters somewhat readily; zircon very slowly indeed. 



While the above statements hold in a general way, besides the con- 

 stituent elements, another very important factor enters into the rate of 

 decomposition, viz, the acidity of the mineral. Thus the feldspars vary 

 from orthosilicates to trisilicates. On this account the orthosilicate anorthite 

 decomposes more readily than the trisilicates albite and orthoclase, not- 

 withstanding the fact that alkalies are absent in the first and abundant in 

 the second. For the same reason some of the basic feldspars decompose 

 under some circumstances more readily than certain of the more acid 

 pyroxene-amphibole and biotite-muscovite groups 



The more abundant relatively insoluble products of the first stage 

 of the alteration of the anhydrous silicates may be classified into hydrous 

 silicates and oxides. The hydrous silicates comprise (1) the kaolin group, 

 (2) the serpentine-talc group, (3) the chlorite group, (4) the hydro-mica 

 group, (5) the zeolite group, and (6) the epidote group. The oxides 

 include (1) the gibbsite-corundum group, (2) the quartz group, and (3) the 

 ferric iron group. 



Under long-continued favorable conditions of the belt of weathering 

 many of these hydrous silicates, which form in the belt of weathering or 

 which migrate into the belt of weathering from the belt of cementation, 

 are further modified and changed to simpler compounds. The hydrous 

 silicates thus further altered comprise serpentines, chlorites, zeolites, and 

 epidotes. These silicates, it may be noted, are minerals very characteristic 

 of the belt of cementation. The serpentine alters into magnesite, quartz, 

 iron oxide, and other compounds. The chlorite passes, into gibbsite, 

 magnesite, iron oxide, and quartz. The epidote passes into gibbsite and 

 quartz. It is probable that the zeolites pass into kaolin, gibbsite, carbon- 

 ates, and quartz. Of course, under the conditions of the belt of weathering, 

 the carbonates, including most of the magnesite, are dissolved and 

 removed. 



