520 A TREATISE ON METAMORPHISM. 



Of the oxides in the original rocks, quartz is that of dominant impor- 

 tance; but hematite and magnetite are of consequence. Quartz is only 

 slowly dissolved. Hematite may become slowly hydrated. Magnetite may 

 be oxidized and hydrated. All of the iron oxides are very slowly soluble. 



It therefore appears that the important end products in the belt of 

 weathering, whether the original compounds are silicates, hydrous or anhy- 

 drous, or are oxides, are included in the following groups: (1) Kaolin 

 group, (2) talc group, (3) gibbsite-corundum group, (4) quartz group, (5) 

 ferric iron group, hydrous or anhydrous. These are the minerals which 

 have become adapted to their environment and- may therefore persist 

 indefinitely, so long as the conditions remain those of the belt of weathering. 

 The elements which constitute this group are but six in number. In order 

 of probable abundance they are as follows: Oxygen, silicon, aluminum, 

 iron, magnesium, hydrogen. 



A sixth unimportant group is formed by the very difficultly decom- 

 posable minerals already mentioned — staurolite, tourmaline, andalusite, 

 cyanite, fibrolite, and zircon, and to these should probably be added the 

 additional rare difficultly decomposable minerals, such as diamond, casit- 

 erite, gold, silver, xenotime, monazite, etc. 



The quartz is largely that which was originally present in the rocks as 

 quartz. Watson notes this fact for the granitic rocks of Georgia. While 

 the total loss of silica in the rocks varies from 10 to nearly 80 per cent, the 

 residual products contain quartz granules similar to those in the original 

 rocks, with little or no evidence of corrosion. a The loss is due to the 

 decomposition of the silicates and the abstraction of the greater part of the 

 liberated silica, as explained on pages 516-517. For the most part the 

 other important minerals of the end series are secondary. 



The foregoing minerals, by the processes of decomposition and solu- 

 tion, are concentrated in the belt of weathering in various proportions. 

 Under one set of conditions one group of them may be dominant and others 

 entirely subordinate. Ordinarily, for instance, the rare difficultly decom- 

 posable minerals mentioned are not noticeable, but if the original rocks 

 contain an unusual amount of them they may become rather important. In 

 some cases a single one of these minerals may become important or domi- 

 nant. An excellent illustration of sueh a case is that of Iron Mountain, 



"Watson, Thos. L., Weathering of granitic rocks of Georgia: Bull. Geol. Soc. America, vol. 12, 

 1901, p. 99. 



