In the magmas of this series the percentage of silica is much less than in the preceding groups (40 to 55%), and the quantity of alkalies is small, while that of iron, magnesia and lime is much greater. They are heavy, dark-coloured rocks. The principal minerals are a plagioclase felspar, rich in lime (labradorite or anor-thite), some kind of pyroxene, magnetite, and frequently olivine. There is a wide range of mineralogical composition and many varieties of rock occur in this family.
Tachylyte is a basaltic glass, which is not at all common.
Basalt is a name of wide application covering many varieties. The basalts are very common volcanic rocks, and most of the active volcanoes of the present day extrude basaltic lavas. In texture the basalts are ordinarily porphyritic, but they may be without pheno-crysts, and consist of a finely crystalline mass. The ground mass is made up of tiny crystals, mingled with a dark glass.
The basalts are closely related to the andesites and connected with them by a number of transitional forms, but in the andesites the phenocrysts are principally felspars, which is not the case in the basalts. Those basalts which contain olivine in notable quantities are called olivine basalt; while those in which the felspar is replaced by leucite or nepheline are called leucite and nepheline basalt, respectively.
Trap is a useful field name for various sorts of dark, granular rocks, which cannot readily be distinguished by inspection. The term is often applied to diorite and especially to diabase.
Dolerite is a coarsely crystalline basaltic rock, which is either porphyritic or granitoid in texture.
Diabase is a rock of peculiar texture; the felspar crystals are long, narrow, and lath-shaped, and contain the dark minerals in their interstices. The trap rocks of the Palisades of the Hudson, and many localities in the Connecticut valley, New Jersey, Maryland, Virginia, and North Carolina, are diabase.
Gabbro is a term which is now used comprehensively to include the coarse-grained, plutonic phases of the various basaltic rocks, which are typically composed of plagioclase and pyroxene. Olivine gabbro and hornblende gabbro are names that explain themselves. Norite, or hypersthene gabbro, contains orthorhombic pyroxene. Anorthosite is nearly pure labradorite in large crystals, with little or no pyroxene; great masses of it occur in Canada and the Adirondack Mountains of New York. Gabbros are present on a great scale in the Adirondacks, and occur in the White Mountains, on the Hudson, near Baltimore, around Lake Superior, in California and various parts of the West.