CEMENTING SUBSTANCES. 623 



and openings in the brecciated rocks. The amount thus deposited is onl}- 

 second to that deposited in the sediments. Finally, as shown in the 

 succeeding section on metasomatism, there are very extensive replacement 

 deposits of quartz. There is no means by which the amount of replace- 

 ment silica can be measured, but the quantity is certainly very great. 



The foregoing facts give us some idea of the vastness of the amount 

 of siliceous material which is deposited in the belt of cementation. 

 Certainly the amount is to be measured in thousands and probably in 

 hundreds of thousands of cubic kilometers. An adequate source of this 

 silica is that liberated by the process of carbonation in the belt of weath- 

 ering. (See p. 480.) 



iron oxides.— Iron oxide is extensively deposited between the grains of 

 mechanical sediments, and therefore is an important cementing agent Iron 

 oxide is also deposited in the other classes of openings in rocks, and also as 

 a replacement product. Iron oxide is deposited as magnetite, hematite, 

 limonite, and other hydrated oxides. 



A very large amount of the iron is precipitated as hematite or limonite 

 or other hydrated oxide. This precipitation is likely to occur where oxygen 

 is abundant. These conditions are likely to obtain where solutions from 

 different sources are united in some main channel of descending ground 

 water. No better illustration of precipitation of iron oxide by this process 

 can be given than the great hematite deposits of the Lake Superior region, 

 which are precipitated by the mingling of waters bearing iron carbonate 

 and those bearing oxygen. (See pp. 1194-1197.) 



Magnetite is deposited in the belt of cementation as a very widespread 

 constituent. The explanation of the precipitation of some magnetite is: In 

 the belt of cementation there is usually a deficiency of oxygen. If the 

 iron be supposed to be carried in the form of carbonate, the reaction for 

 the precipitation of magnetite may be written as follows: 



3FeC0 3 +0=Fe 3 4 +3C0 2 . 



Other methods by which magnetite is precipitated are considered on 

 pages 845-846. Where iron is thrown down as magnetite it is ordinarily 

 found in crystals. As illustrations of this may be mentioned formations in 

 the Lake Superior region, some of which are of economic importance, as, 

 for instance, the magnetite of the Michigamme, Republic, and other mines 



