CEMENTING SUBSTANCES. 627 



are deposited usually contain these same minerals in abundance, and in 

 many cases the formations extend to the surface. The materials for the 

 solutions may have been derived from the same minerals in the belt of 

 weathering, or from a part of the belt of cementation above the places 

 of deposition in case the waters are descending, and below the places of 

 deposition in case the waters are ascending. 



SULPHIDES. 



The only important sulphide cement is that of iron. Iron is somewhat 

 extensively thrown down as pyrite or marcasite. This may be precipitated 

 by means of hydrosulphuric acid; it may be produced by the reduction of 

 iron sulphate by means of organic reducing agents in the solutions or the 

 rocks. These and other methods of precipitating sulphides are fully con- 

 sidered on pag-es 1108-1118. The quantitative importance of the cement- 

 ing sulphides in the belt of cementation, from a geological point of view, is 

 small, but from the point of view of ore deposits they are of the greatest 

 consequence. (See pp. 1104-1105.) 



DISTRIBUTION OP ELEMENTS IX CEMENTING MINERALS. 



From the foregoing it appears that the elements added to the solutions 

 from the belt of weathering and from the belt of cementation itself are 

 distributed between the various minerals as follows: 



The sodium passes into the silicates as zeolites, and may produce any 

 one of the following sodium-bearing minerals of this group: Thomsonite, 

 hydronephelite, natrolite, mesolite, gmelinite, analcite, phillipsite. The 

 potassium passes into silicates as zeolite, being' a constituent of apophyllite. 

 The calcium passes into the carbonates and silicates. As a carbonate it is 

 a constituent of calcite, dolomite, ankerite, and parankerite. As a silicate 

 it enters into the zeolites and epidotes. The zeolites containing calcium 

 are thomsonite, mesolite, scolecite, apophyllite, stilbite, phillipsite, gis- 

 mondite, chabazite, and laumontite. Both epidote proper and zoisite con- 

 tain calcium. The magnesium may enter into carbonates or silicates. As 

 a carbonate it is a constituent of dolomite, ankerite, and parankerite. As a 

 silicate it is a constituent of all the chlorites, of serpentine, and of talc. 

 The iron may pass into oxide, carbonate, silicate, or sulphide. As an oxide 

 it is a constituent of magnetite, hematite, and limonite. As a carbonate 

 it is a constituent of ankerite, parankerite, and siderite. As a silicate it is 

 a constituent of epidote and chlorite. As a sulphide it is a constituent of 



