Certain minerals are as a matter of common observation not found together in igneous rocks. Thus olivine (except fayalite) and quartz do not occur in association, but are represented by rhombic pyroxene. The associations nepheline and quartz, leucite and quartz, pyroxenes or amphiboles and muscovite, are almost unknown among magmatic crystallizations; on the other hand we find certain mineral associations or parageneses to occur fre quently; thus aegirine and arfvedsonite, nepheline and nosean, common hornblende and titanite, are well known as faithful com panions in igneous rocks. Again some minerals are restricted to particular magmas; melanite, melilite and certain rare zirconium titanium minerals are almost confined to the alkaline igneous rocks. By inspection of a rock analysis it is frequently possible to state approximately what minerals the rock will contain, but there are numerous exceptions to any rule which can be laid down. The minerals which crystallize from an igneous magma may depend largely on the physical conditions under which the liquid consolidates; accordingly we find rocks of almost identical chemical composition with widely different mineralogical constitu tion. Such rocks as already noted are referred to as hetero morphic. Some familiar examples of assemblages of this char acter are mica syenite and leucite basalt, minette and leucite basalt, pyroxenite and allivalite, monzonite and leucite tephrite, hornblendite and websterite.
Certain minerals are practically confined to deep seated rocks, e.g., microcline, muscovite, diallage and almandine garnet ; on the other hand leucite is very rare in plutonic rocks, as is also anortho clase and the variety of potash felspar known as sanidine. Some of these peculiarities are readily explicable, being dependent on the pressure, temperature, or concentration of volatile constituents in the crystallizing magma. Others still remain as a problem for solution. The subject will be referred to again in a later part of this article.
In the classification generally adopted by petrographers, the silica content of the magma forms a basis of further subdivision of igneous rocks. The division of volcanic rocks into acid, inter mediate and basic we owe in the first place to Abich (1841). As applied to igneous rocks in general, those which contain most silica and which if crystallized yield free silica as quartz, are erected into a group designated the "acid" rocks in allusion to the role of silica in the mineral kingdom. Those with low silica percentages and rich in magnesia, lime and iron—so that quartz is absent while olivine, pyroxene or calcic felspar is usually abundant—form the basic group. The "intermediate" rocks in clude those characterized by the general absence of both quartz and olivine. An important subdivision of the intermediate as well as the basic group contains a very high percentage of alkalis (soda or potash or both), and consequently has minerals such as nepheline, nosean, analcime or leucite, minerals not common in other rocks. This group is often referred to as the alkaline
rocks. Lastly a small subgroup rich in olivine (peridotite, dunite), pyroxene (pyroxenite), or calcic felspar (anorthosite) have been called the "ultrabasic" rocks. They have low percentages of silica but much iron, magnesia or lime. With the exception of the last group, including also ultrabasic li.vas as augitite and limburgite and a few related rocks as monchiquite, melilite basalt and alnoite, practically all igneous rocks contain felspar or felspathoid min erals. In the acid rocks the common felspars are orthoclase, with perthite, microcline and oligoclase, all having much silica and alkalis. In the basic rocks labradorite, bytownite and anorthite prevail, being rich in lime and poor in silica, potash and soda. Augite and olivine are the common ferromagnesian minerals of the basic rocks, but hornblende and biotite are on the whole more frequent in the acid rocks.
The classification in the two tables that follow is based essen tially on the mineralogical constitution of the three great groups of igneous rocks. The alkali rocks containing nepheline and leucite are treated separately, for though they show transitions into the corresponding intermediate and basic rocks (syenites, diorites, gabbros, etc.), they contain many minerals which are unknown in other rocks. In a purely formal classification such as is out lined here they are more conveniently considered as a distinct series. Their genetic relations among themselves and with the other rock types are considered later.
Diaschistic Rocks.—Two groups of rocks not specifically included in the tables form important members of the hypabyssal division of igneous rocks. These are the aplites and the lampro phyres. The aplites are acid leucocratic differentiation products of the granites, syenites, diorites and gabbros, while the lampro phyres are basic differentiates. The two groups are together known as complementary or diasc/iistic rocks and usually occur as dikes or sills in association with the parent rock from which they are derived. Thus the minettes, vogesites and kersantites (see LAM PROPHYRE) are complementary to granitic aplites and occur in association with granitic masses. Camptonites and bostonites (syenite aplites) have a parent magma in the essexites theralites which they accompany.
The classificatory scheme here adopted is admittedly artificial for rocks are subdivided without particular reference to their genetic relationships. Even regarded as a classification for a utilitarian end there are many imperfections ; the subdivisions are of unequal value and transitional types are not included. The latter however can be accommodated by further subdivision. Many of them have received special names. The quartz syenites may be interposed between granite and diorite, the tonalites be tween granite and diorite and so on.