The diorites.—These embrace rocks which were crystallized from a magma still poorer in silica and the alkalies, and richer in the earthy bases. In composition they closely approach the ideal average rock, but usually fall a little below it in silica and the alkalies, and rise a little above it in the earthy bases. In current usage, diorite is defined as an intimate mixture of crystals of hornblende and a plagioclase feldspar. It differs from the syenite in having plagioclase feldspar instead of orthoclase. By substitutions and the addition of accessory minerals, the diorites graduate toward the granites and syenites on the one hand, as already noted, and into gabbros on the other.
In the scheme recently proposed, all holocrystalline rocks in which hornblende is dominant and feldspar subordinate are classed as diorites.
The gabbros.—The name gabbro was formerly applied to a coarse-grained basic rock consisting of labradorite and diallage, but the name has been gradually extended until it embraces a large group of rocks that have essentially the same composition as the dolerites mentioned below, but are coarser in crystallization, and the crystals do not embrace one another (i.e., are not ophitic). The principal minerals are plagioclase (normally labradorite) and pyroxene (normally diallage) with magnetite or ilmenite. They are usually dark, heavy rocks. The pearly luster of the cleavage faces of the diallage, when present, gives a peculiar sheen to a fresh surface of the rock. In the recently proposed field names, gabbro is made to include all phanerocrystalline rocks in which pyroxene predominates, attended by feldspar of any kind in subordinate quantity, with or without hornblende or mica.
The peridotites.—These stand at the basic end of the series, having been formed from a magma in which the silica was low (39–45 per cent.), as were also the alumina, lime, and alkalies, but in which the magnesia was relatively very high, ranging from 35 to 48 per cent. The rock consists very largely of olivine associated with pyroxene, magnetite, and other very basic minerals. Little or no feldspar is present. The peridotites are much less abundant than the preceding classes and represent a very distinctive phase of the magma in which the magnesia was greatly concentrated.
Closely allied to the peridotites are rocks which are made up largely of a single basic mineral, as augitite, pyroxenite, hornblendite, rocks essentially formed of the minerals augite, pyroxene, and hornblende respectively. It will be noted that in these rocks the magma became quite simple in nature, just as at the acid end of the series certain rocks become comparatively simple from the concentration of the acid element, as in certain acidic granites, felsites, etc. (See [pp. 523–524].)
The basalts.—The term basalt is used in a somewhat comprehensive way to embrace dark, compact, igneous rocks that appear to be nearly homogeneous, owing to the minuteness of the crystals, which are usually so small as to be identifiable only under the microscope. In some cases the crystals are scattered throughout a ground-mass after the porphyritic fashion. In some of these cases there is a true glassy base, and in such cases the rock does not strictly belong in the holocrystalline group. In the more typical cases the constituent minerals are very minutely crystallized and intimately intermixed. The leading minerals are plagioclase (usually labradorite or anorthite) and pyroxene (usually augite), with olivine and magnetite or ilmenite usually present. There is a considerable range in chemical nature, but the basalts are relatively poor in silica, usually also low in potash and soda, but rich in lime, magnesia, and the iron oxides. They are classed as basic and are sometimes highly so. The magmas of the basalts are especially fluid, and when poured forth upon the surface easily spread out in thin sheets. In cooling they are prone to take on a columnar or basaltic structure, the columns standing at right angles to the surfaces exposed to cooling. The columns are sometimes curved, owing to the peculiar attitude of the cooling surface. The columns of Giant’s Causeway and Fingal’s Cave are familiar examples.
The dolerites.—The basalts graduate insensibly into the dolerites; indeed the dolerites may be regarded simply as basalts of coarser crystallization. The minerals are evident to the eye and range up to medium size. The more abundant minerals are plagioclase feldspar (labradorite or anorthite), with one or more of the ferromagnesian minerals (augite, olivine, or biotite), and magnetite or ilmenite. In the growth of the minerals one crystal frequently embraces others, giving an ophitic structure. The dolerites have many varieties, due either to accessory minerals or to the development of some of the constituents more amply than the rest. The type may be said to consist of plagioclase and augite, the other minerals being regarded as accessories. Magnetite or ilmenite is almost universally present. The varieties are usually designated by prefixes, as olivine-dolerite, enstatite-dolerite, etc., but special names are also used for some of these.
Fig. 346.—Conglomerate, Carboniferous series. Bancroft Place, Newport, R. I. (Walcott, U. S. Geol. Surv.)
The ancient dolerites have usually undergone internal changes and such rocks are often called diabases. While the use of the term has not been uniform, it accords with the better practice to regard the diabases simply as partially altered dolerites and basalts. In general, therefore, the diabases are but ancient dolerites.