Habit of Igneous Rocks

Plutonic or Abyssal Types.

Let us now consider the char acteristics of a typical deep-seated rock similar to granite or diorite. That these are clearly igneous is proved by the manner in which they have burst through the superincumbent strata, filling the cracks with ramifying veins; that they were at a very high temperature is equally clear from the changes which they have induced in the rocks in contact with them. But as their heat could dissipate only very slowly, because of the masses which covered them, complete crystallization has taken place and no vitreous rapidly chilled matter is present. As they have had time to come to rest before crystallizing they are not fluidal. Their contained gases have not been able to escape through the thick layer of strata beneath which they were injected, and may often be observed occupying cavities in the minerals, or have occa sioned many important modifications in the crystallization of the rock. Because their crystals are of approximately equal size these rocks are said to be granular; there is typically no distinction between a first generation of large well-shaped crystals and a fine grained ground-mass. Their minerals have formed, however, in a definite order, and each has had a period of crystallization which may be very distinct or may have coincided with or overlapped the period of formation of some of the other ingredients. The earlier have originated at a time when most of the rock was still liquid and are more or less perfect ; the later are less regular in shape because they were compelled to occupy the interspaces left between the already formed crystals which pressed on them. The former are said to be idiomorphic (i.e., having their own char acteristic form, Gr. 1.6tos, belonging to one's self), the latter are allotriomorphic (Gr. aAA67-ptos, belonging to another). There are also many other characteristics which serve to distinguish the members of these two groups. Orthoclase, for example, is the typical felspar of granite, while its modification sanidine occurs in lavas of similar composition; leucite is common in lavas, very rare in plutonic rocks ; muscovite is confined to the intrusives. These differences show the influence of the physical conditions under which consolidation takes place.

Intrusive or Hypabyssal Types.

There is a certain class of intrusive rocks which have risen upwards towards the surface, but have failed to reach it, and have solidified in fissures as dikes and intrusive sills at no great depth. To this type the name hypabyssal is often given in distinction to the plutonic (or abyssal) which formed at greater depths. As might be expected, they show structures intermediate between those of the effusive and the plutonic rocks. They are very commonly porphyritic, not rarely vitreous, and sometimes even vesicular. In fact many of them are indistinguishable petrologically from lavas of similar composition.

The attempt to form a special group of hypabyssal (intrusive and dike) rocks has met with much criticism and opposition. Such a group certainly cannot rank as equally important and equally well characterized with the plutonic and the effusive. But there are many kinds of rock which are not found to occur normally in any other manner. As examples we may cite the lamprophyres,

the aplites and the porphyries. These never occur as lava flows or as great plutonic bosses ; if magmas of the same composition as these rocks occur in either of these ways they consolidate with different assemblages of minerals and different structures.

Subdivisions of Igneous Rock Class.

In subdividing the plutonic, the hypabyssal and the effusive rocks, the principle is followed of grouping those together which resemble one another in mineral constitution and in chemical composition. In a broad sense these two properties are interdependent.

Chemical Characters.

Twelve elements (oxygen, silicon, aluminium, iron, calcium, sodium, potassium, magnesium, titan ium, phosphorus, hydrogen and manganese) constitute about 99.6% of the earth's crust. Of these oxygen is the most abundant and constitutes 46.59% of the igneous rocks of the crust, the elements given above being placed in their order of abundance.

These "petrogenic" or rock elements occur typically as oxides, simple silicates, aluminates, fluorides, chlorides and sulphides. In rock analyses they are stated as oxides. From a computation based on 5,159 analyses Washington and Clarke have arrived at the following as the average percentage composition of all igneous rocks :--Si02, 59.12 ; A1203, Fe203, 3-08; MgO, 3.49; CaO, 5.o8; Na20, ; H20, 1.15; Ti02, 1.05; P205, 0.3; MnO, 0.1 2 ; Inclusive 0.5.

The known igneous rocks show a wide but limited range of composition. As the most abundant and essential rock minerals are either silica or silicates, silica shows the highest maximum and the widest range of these oxides. In some magmatic ores its con tent falls to zero but rises to 93-96% in rare igneous rocks (north fieldite) ; alumina reaches a maximum of about 6o% in some corundum-bearing syenites; iron oxides have a general range from 15 to under i%, and magnesia from 25 to under 1 %. In some olivine rocks (dunite) it reaches a value of 48%. Lime is highest in some pyroxenites and anorthosites (2o-22%), but its general range is from 15% to zero. Soda reaches a maximum of 2o% in almost pure nepheline rocks (congressite) but has a general range from 15% to nearly zero. Potash has a maximum 18% in an almost pure leucite lava (italite), but usually varies from io% to zero. Water in fresh crystalline igneous rocks seldom reaches more than 2% but glassy lavas may contain 1 o%.

The commonest minerals of igneous rocks are the felspars, pyroxenes and amphiboles, quartz, and the micas. From a sta tistical examination of 700 igneous rocks an average composition has been calculated as follows, felspars 59.5, pyroxenes and amphiboles 16.8, quartz 12.0, micas 3.8, other minerals 7.9 (apatite o.6, titanium minerals 1.5). This estimate however is clearly only an approximation. Rock minerals have been classified empirically into two groups, according as they are capable or incapable of existing in the presence of free silica, namely saturated and un saturated minerals. The former group includes such minerals as the felspars, pyroxenes, amphiboles and micas; and the latter olivine, nepheline, leucite and other felspathoid minerals.