CEMENTING SUBSTANCES. 625 



as quartz is enlarged. The deposition of magnesium carbonate in many 

 cases involves the solution of an equivalent amount of calcium carbonate, 

 in which case the process is that of metasomatism, fully considered on 

 pag-es 640-646. 



The quantity of calcite and dolomite deposited between the grains of 

 sediments, the vacuoles of igneous rocks, the openings of fissility, joints. 

 and faults, while very great, is vastly less than that of quartz. 



siderite. — The iron in solution may be partly or largely precipitated in the 

 form of siderite, ankerite, or ferrodolomite. These compounds all form 

 somewhat extensively in the minor interstices in rocks, and occasionally* 

 they may form veins of such magnitude as to be worked as iron ore. 



SILICATES. 



The silicates which are most abundantly precipitated as cements are 

 hydrous, but anhydrous silicates are also deposited. The important 

 cementing silicates include, in order of abundance, the (1) zeolites and 

 prehnite, (2) chlorites, (3) epidotes, (4) serpentine and talc. The zeolites 

 and prehnite are sodium-aluminum silicates, calcium-aluminum silicates, 

 sodium-calcium aluminum silicates, and potassium-calcium silicates. The 

 chlorites are magnesium-aluminum silicates when pure, but ordinarily the 

 magnesium is partly replaced by iron. The epidotes comprise zoisite 

 (calcium-aluminum silicate) and epidote (calcium-aluminum-iron silicate). 

 Serpentine and talc are magnesium silicates. As to the degree of hydra- 

 tion, the minerals may be divided into three classes: The zeolites and 

 prehnite, the chlorites and serpentine, and epidote and talc. The bases 

 of all of the above compounds are those which abundantly enter into the 

 solutions. Hydration, as already shown, is one of the most important of 

 the chemical processes which result in the liberation of heat. Probably 

 the dominant factor in the precipitation of these hydrous compounds is the 

 chemical law obtaining in the zone of katamorphism that, other things 

 being equal, those compounds form by which the greatest amount of heat 

 is developed and liberated. 



The anhydrous silicates which are somewhat abundantly deposited in 



the belt of cementation are feldspar, hornblende, and mica, the order given 



being that of relative abundance. These materials may be deposited as 



independent constituents in the interstices of the rocks, and they may also 



mon xlvii — 04 40 



