THE PHANEROCRYSTALLINE ROCKS.
The phanerites.—When time enough is given for the cooling process the molten magma becomes completely crystalline. The holocrystalline rocks hence include a large series, ranging from the most acid to the most basic. In this class the differentiation of the rock material and the formation of distinct minerals reach a high stage, and as a natural result the varieties of rock are numerous. Taken as a group they are phanerites. If they are to be more particularly characterized, it is usually done on the basis of the minerals of which they are composed. The following are the leading types, beginning with those which are rich in silica and poor in basic oxides, and ending with those which are rich in basic oxides and poor in silica.
Fig. 344.—Granitic texture. About half natural size. (Photo. by Church.)
The granites.—The term granite was originally used to designate a granular, i.e., a distinctly crystalline, rock, and it is still popularly and properly so used. In scientific treatises it has usually been confined to a special aggregate of crystals of quartz, feldspar, and mica. It has recently been proposed to give it again a more general application, though not quite its original one, by including under it all holocrystalline rocks composed of dominant quartz and feldspar of any kind, with mica, hornblende, or other minerals in subordinate amount. In scientific literature as it now stands, granite consists of quartz, feldspar, and mica, the feldspar being of the alkali-potash or soda variety (orthoclase, microcline, or albite), and the mica, either muscovite or biotite. In the type form the crystals are distinct and sometimes large ([Fig. 344]). They are intimately mingled with one another, and in growing, interfered more or less with each other and so became interlocked. The granites are among the most common and easily recognized of the holocrystalline rocks. Their color is mainly dependent upon the feldspar, the red and pink varieties of the mineral giving rise to red granite, and the white varieties to gray granite.
Fig. 345.—Graphic granitic (or pegmatitic) texture. Nearly natural size. (Photo. by Church.)
Very few granites conform strictly to the type. They vary by the addition and substitution of other minerals, and these sometimes become as prominent as the type minerals. The soda-lime feldspars sometimes take the place of the orthoclase, or accompany it; hornblende and other minerals take the place of the biotite, or occur with it; and so on. Whenever one of these replacing or accessory minerals is notable in quantity, its name is often prefixed, as hornblende-granite, oligoclase-granite, zircon-granite, etc. In this way the rock grades almost insensibly into the syenites, diorites, etc. Variations also arise from the absence of one of the three leading minerals. If mica is absent, the rock is termed an aplite (quartz and feldspar). If the feldspar is absent, it is called a greisen (quartz and mica). If quartz is absent, it is termed a minette (feldspar and mica). These varietal terms are neither universally nor always consistently used, and it is to be hoped they will be replaced by the systematic nomenclature recently proposed and outlined later ([p. 451]).
The granites were formed from a magma rich in silica, alumina, potash, and soda, but generally poor in lime, iron, and magnesia. Incidentally other substances were present. The alumina, potash, and other bases united with so much of the silica as was required to form the feldspars and micas, and the remaining silica crystallized into quartz.
Granite is normally a massive rock without foliation or banding. If it takes on these characters, it becomes a gneiss, and passes into the foliated or schistose class of rocks, to be discussed later. The texture of graphic granite (see pegmatite) is notably peculiar, due to the simultaneous crystallization of the quartz and feldspar ([Fig. 345]).
The syenites.—When the mica of a granite is replaced by hornblende, the rock is now commonly known as a hornblende-granite, but it was formerly called syenite, because found at Syene on the Nile. The term syenite is now applied to a rock consisting essentially of feldspar and hornblende or mica, but there is a complete gradation from the granites to the syenites. The magma of the syenites was richer in iron and magnesium than the typical granitic magma. The syenites also grade into other classes, as do the granites, and are named by similar prefixes, as augite-syenite, etc., and some of these varieties have special names. The syenites are red or gray, according to the color of the feldspar, and are usually darker than the granites. The texture of syenite is like that of granite. In the scheme of field names recently proposed, syenite is made to include all holocrystalline rocks composed mainly of feldspar of any kind, with subordinate amounts of mica, hornblende, pyroxene, and other minerals, but without a noticeable amount of quartz.
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.
General names.
The difficulty of distinguishing many of the foregoing rocks from each other by any means available in the field, owing to the minuteness of the crystals, and to the gradation of one type of rock into another, makes it desirable to employ certain general names which will correctly express the leading character of the rock without implying a knowledge of the precise mineral composition. A convenient term of this kind is greenstone, which merely indicates that the ferromagnesian minerals are prominent and usually give a greenish or dark cast to the rock. The greenstones embrace the diorites, dolerites, some of the gabbros and the basalts, and may even extend to the peridotites and some of the more hornblendic of the granitoid rocks. Another convenient name is trap, which may be used for any dark, heavy igneous rock. The name (from trappe, stairs) refers to the step-like arrangement which the edges of the superimposed sheets of lava often take, especially when the lava is of the free-flowing, basaltic kind.
Fig. 347.—Brecciated limestone, Calciferous formation. One mile south of Highgate Falls, Vt.
The term basalt is sometimes used to embrace any of the very fine-grained dark igneous rocks. In such cases, it covers the very fine-grained dolerites, diorites, peridotites, etc. The term granite was used originally for any coarse-grained crystalline rock, and there is a tendency to revive this early use. In general descriptions, some of our best petrographers call any coarsely crystalline rock (e.g., coarse-grained syenites, diorites, gabbros, etc.) granite. The term granitoids may be used with strict propriety to cover all rocks of this class.