STEATIFIED OE BEDDED 141 



may be divided into two general groups : 1. Stratified or bedded ; 

 2. foliated or schistose. 



1. STEATIFIED OE BEDDED 



(1) THE CEYSTALLINE LIMESTONES AND DOLOMITES 



Here are included the metamorphosed form of the sedimentary 

 rocks described on p. 125. 



Mineral Composition. — The essential constituent of the crys- 

 talline limestones is the mineral calcite. The common acces- 

 sories are minerals of the mica, amphibole, or pyroxene group, 

 and frequently sphene, tourmaline, garnets, vesuvianite, apatite, 

 pyrite, graphite, etc. 



Chemical Composition. — As may be inferred from the mineral 

 composition, these rocks, when pure, consist only of calcium 

 carbonate. They are, however, rarely if ever found in a state 

 of absolute purity, but show more or less magnesia, alumina, 

 and other constituents of the accessory minerals. The analyses 

 given on p. 127 will serve equally well here, and need not be 

 repeated. 



Structure. — The limestones are eminently stratified rocks, 

 though this peculiarity is not always sufficiently marked to be 

 seen in the hand specimen. The purest and finest crystalline 

 varieties often show a granular texture like that of loaf sugar, 

 and hence are spoken of as saccliaroidal limestones. Statuary 

 marble is a good illustration of this type. Under the micro- 

 scope the stone is shown to be made up of small grains, which, 

 having mutually interfered in process of growth, do not possess 

 perfect crystal outlines, but are rounded and irregular in out- 

 line, as shown in Fig. 14. All grades of textures are common, 

 the coarser forms sometimes showing individual crystals an inch 

 in length. Though in their unchanged conditions highly fossil- 

 iferous or tufaceous, these structural features may be wholly 

 or in part obliterated by crystallization. 



Colors. — The color of pure limestone is snow-white, as seen 

 in statuary marble. Other common colors are pink or reddish, 

 greenish, blue-gray through all shades of gray to black. The 

 pink and red colors are due to iron oxides, the greenish as a 

 rule to micaceous minerals, the blue-gray and black to carbon- 

 aceous matter. 



