──────────────────────────┼──────────────┼───────────────────────────── Acidic Group: │ │ Granite │ 2.58 to 2.73 │ 77.65 to 62.90 Liparite │ 2.53 to 2.70 │ 76.06 to 67.61 Obsidian │ 2.26 to 2.41 │ 82.80 to 71.19 Obsidian pumice │ Floats on │ 82.80 to 71.19 │ water. │ │ │ Intermediate Group: │ │ Syenite │ 2.73 to 2.86 │ 72.20 to 54.65 Trachyte │ 2.70 to 2.80 │ 64.00 to 60.00 Hyalotrachyte │ 2.4 to 2.5 │ 64.00 to 60.00 Andesite │ 2.54 to 2.79 │ 66.75 to 54.73 │ │ Basic Group: │ │ Diabase │ 2.66 to 2.88 │ 50.00 to 48.00 Basalt │ 2.90 to 3.10 │ 50.59 to 40.74 Peridotite │ 3.22 to 3.29 │ 42.65 to 33.73 Peridotite (iron rich) │ 3.86 │ 23.00 Peridotite (meteorite) │ 3.51 │ 37.70 ──────────────────────────┴──────────────┴───────────────────────────── =31. Color Of Rocks.=—The color of rocks is determined chiefly by the oxids of metals which they contain and the degree of oxidation of the mineral in each particular case. There are, however, many colors of rocks which seem to depend not upon any particular mineral ingredient which they contain, but upon some particular crystalline structure or physical condition. The chief coloring matters in minerals are those which form colored bases such as iron, manganese, chromium, etc. The yellow, brown, and red colors, common to fragmental rocks, are due almost wholly to free oxids of iron. The gray, green, dull brown, and even black colors of crystalline rocks are due to the presence of free iron oxids or to the prevalence of silicate mineral rich in iron, as augite, hornblende, or black mica. Rarely copper and other metallic oxids than those of iron are present in sufficient abundance to impart their characteristic hues. As a rule, a white or light-gray color denotes an absence of an appreciable amount of iron in any of its forms. The bluish and black colors of many rocks, particularly the limestones and slates, are due to the presence of carbonaceous matter. In still other cases, and particularly the feldspar-bearing rocks, the color may be due in part to the physical condition of the feldspar. Inasmuch as the color of rocks is due so largely to metallic oxids, it is easy to see that they may undergo changes when exposed to weathering, or the degree of oxidation may change, and either, together with changes in the physical structure of the rock, may cause a distinct change in color. Luster is often considered in connection with color, and is due almost exclusively to physical conditions.