Page:EB1922 - Volume 32.djvu/100

82

Descriptive petrology has been by no means in abeyance, though the five years of war turned the activities of many geologists to other fields. Exploring expeditions, such as those of Scott, Shackleton and Bruce in the Antarctic, have brought home large collections of rocks which have been examined and described, and the constant activities of geological surveys in all parts of the world, together with the researches of geological specialists, have added largely to our knowledge.

Igneous Magmas. Of the three great groups into which rocks are naturally subdivided, the sedimentary, igneous and metamorphic, the first is on the whole best understood and presents the smallest number of unsolved difficulties. The man- ner in which sediments are laid down on the bottoms of seas and lakes, in river deltas and valleys and on land surf aces, at the pres- ent time is open to investigation by simple means, and, except in the case of the deposits of the deeper parts of the oceans, is reasonably clear. Igneous magmas, on the other hand, are essentially obscure in their origin and history, and they have been the subject of much investigation in recent years. The origin of magmas is a problem belonging to geology rather than petrology. They have been regarded as unconsolidated rem- nants of the primeval molten globe, which by geological changes, such as the secular contraction arising from cooling and the pressures by which mountain chains have been upheaved, have been afforded an outlet to the surface, where they appear as volcanoes, or have been forced between the rocks of the upper layers of the earth's crust where they may be laid bare by subse- quent denudation as " bosses " of granite or gabbro or intrusive sills of porphyry or dolerite. Others have held that magmas may arise in whole or in part by the fusion of solid rocks (of any of the above three classes) ; the agencies producing fusion being rise of temperature, either through crushing and movement or by de- pression into those regions where a high temperature naturally prevails, or through penetration of gases from the earth's interior which are not only intensely hot but are capable of combining and setting free large quantities of energy. It is conceivable also that deep within the earth's crust masses of rock occur at tempera- tures so high that, if pressure be relieved by the vaulting-up of the overlying crust or by fissures opening to the surface, they may become liquid and rise through any available channels.

Hardly less obscure than the origin of magmas is the question of their variety or differentiation. The outstanding fact in this connexion is that no large developments of igneous rock are really homogeneous and even in small masses it is frequent to find that a great number of varieties or rock-species occur, differing in their chemical composition and their minerals. Granite, diorite, gab- bro, norite and peridotite may all occur within a small outcrop not more than one or two square miles in area. The origin of differentiation has been much discussed. Some have ascribed it to diffusion or to a principle by which the heavier atoms in the molten mass will be concentrated either towards the bottom or towards the cooler edges or surfaces of contact with thesurround- ing rocks. Along these lines no satisfactory explanation has been found. More recent speculations have followed three lines: (a) subsidence or flotation of crystals; (b) absorption of sedi- mentary or other foreign rocks; (c) concentration of vapours.

(a) Subsidence or Flotation of Crystals. When crystals form in a liquid they will, if heavier than the liquid, tend to subside and be collected near the base; and if lighter, they will tend to rise. If the crystals differ in composition from the liquid, as they usually do, consolidation will result in a mass which is not homogeneous. Thus, for example, olivine crystallizes early in a basic magma and, being heavy, will tend to sink; consequently magnesia ana iron will be in excess (and silica will be less abundant) at the base, while felspar will predominate towards the top of such a mass. Instances occur showing this arrangement, but they are very exceptional; it is not the case that dolerite and gabbro masses as a rule have a pale- coloured felspathic top and a dark base rich in olivine and the oxides of iron. For such cases as do occur another explanation is often available. If now it were possible at an advanced stage in crystalliza- tion to drain away or force into another position the still liquid part of the magma a type of rock different from the original magma would be produced, because most of the heavy ferromagnesian minerals would have been abstracted.

The explanation is so simple and so well justified by experiment that it is difficult to believe it has no application in the differentiation

of rock types. There is much evidence, however, that forbids us to accept it as important. For example, the study of great dolerite in- trusions by means of bores shows that they are generally nearly homogeneous throughout. Top and bottom are very much the same as a rule; yet these masses must have cooled and crystallized with extreme slowness; and every chance was afforded for the accumulation of heavy crystals in their deeper parts. Again, it is the case that where many varieties of rock occur in one mass the densest are not generally in the lowest horizons. Still more important is the fact that field evidence often shows clearly that the more basic members of a complex intrusion have been injected in a liquid state into an earlier less basic mass already in position. In that case, if the more basic member was produced by the gravitational sinking of heavy crystals these must have been subsequently melted up, a process for which it is very difficult to suggest an explanation.

(b) Absorption of Foreign Rocks. A second method of differentia- tion which has received much attention of recent years is by absorp- tion of country rock. A gabbro mass, for example, may be sup- posed to dissolve a felspathic sandstone with which it is in contact and thus give rise to a more acid magma which might be represented by quartz-dolerite or even by granite. Intrusive masses of igneous rock, as they ascend from beneath, break across the overlying strata and may shatter them into many small blocks. On these and on the surrounding walls a solvent action is likely to take place. If the invaded rock is heavier than the intrusive magma its fragments will tend to sink, and as they are warmed up they will slowly disappear. There can be no doubt that this action is by no means unusual and many good instances of it are known, but there is little reason to believe that it is an important cause of differentiation. Where igneous rocks have absorbed sediment in any quantity they present in general an abnormal facies. Granites, for example, which have dissolved clay, slate or mica-schist usually contain andalusite, silli- manite, cordierite, garnet or corundum, minerals which do not normally occur in such rocks. The magmas are said to be " con- taminated." Gabbros under similar conditions contain cordierite, garnet and an excess of hypersthene (forming cordierite norites) and are easily distinguished from normal gabbros. Absorption of lime- stone by some nepheline syenites is indicated by the presence of crystalline calcite in the igneous rock, and peridotites may con- tain corundum. Perhaps the diamond is an accessory of this type in olivine rocks which have dissolved graphitic matter. Even when igneous rocks are absorbed the result willasa rule be an abnormality. This is fairly evident in the majority of cases merely by a study of the analyses of true igneous rock types and ordinary sediments.

It is to be remembered, however, that a study of the crystallized igneous rocks exposed by denudation of the earth's surface reveals to us merely the last stages in their history when the upward propul- sive force was spent, the fluid mass had come to rest, its temperature had fallen to such a point that crystallization had begun, part of its gases had escaped and the whole mass was encased in a solid envelope which had resulted from the rapid chilling of its external parts in contact with the cold surrounding rocks. At an earlier stage the magma was at a higher temperature and was in active movement. Constant stirring was going on and dissolved matter rapidly scat- tered through the whole mass. In the deeper parts of the crust the solid rocks may be mostly gneisses or old intrusive rocks different in character and in their relative abundance from the sediments of post-Archaean times, richer in alkalis and more nearly akin in com- position to the igneous masses. These conditions would favour ab- sorption and mask the consequences; and if it took place on a very large scale and time were given for diffusion a magma might be produced which would closely mimic a purely igneous magma. It has also been suggested that the absorption of foreign matter might upset the equilibrium of the original magma and give rise to partial magmas incompletely soluble in one another. It would be very difficult in such a case to determine the original nature of the rock or the amount and composition of the material dissolved.

(c) Concentration of Vapours. The theory that differentiation of magmas arises from the formation of partial magmas during cooling, which separated because they became insoluble in one another (as phenol does with water), is favoured by many geologists, but a recent examination of the physical laws determining the production of such magmas has led to the conclusion that nothing is known that would make this process appear likely. Many geologists, however, who have a wide knowledge of igneous rocks in the field, hold that there is evidence to show that differentiation took place before crys- tallization began, and that the various types of rock were already distinct when they were injected in liquid form into the positions they now occupy.

It has been suggested, though it has not been clearly explained, that the gases dissolved in magmas determine the sequence of crystallization and may exert a powerful influence in differentiation. A magma rich in gases when it begins to crystallize yields crystals of anhydrous minerals. The gases, if they do not escape, must in- crease in relative amount in the liquid residuum. The early minerals are those like olivine and augite, which can be crystallized without difficulty from anhydrous melts; the later minerals, such as the alkali felspars and quartz, crystallize readily only in presence of steam and other gases (or of solvents of a nature not usually present in rocks). In some respects the crystallization of an igneous rock