Page:EB1911 - Volume 18.djvu/240

 veins deposited by ascending hot waters. Hence contact aureoles are common sites for mineral deposits of economic value.

In some aureoles the sediments or schists have their bedding and foliation planes wedged apart by the intrusive force of the granite, and are permeated by igneous material invading them along these fissures. In this way a mélange is produced of sedimentary rock with threads and veinlets of igneous nature, and to some extent a blending of the two rocks takes place, though usually each preserves its identity however intimately mixed. In microscopic sections veins of granite not more than a tenth of an inch in width may be traced, sharply distinct from the slate or schist they penetrate. Cases, however, are described in which the rocks of the aureole have been felspathized or filled with new felspar derived from the granite; this, however, is not common. Shales are often converted, when in contact with diabase, into pale-coloured, flinty-looking rocks known as adinoles. These are exceptionally rich in albite and contain as much as 10&#x202f;% of soda, an amount which is not met with in unaltered shales. It seems probable that alkalis have been transferred from the igneous rock to the sedimentary, perhaps through the medium of the vapours exhaled. The breadth of the adinole belt is as a rule only a few inches or a foot or two.

The vapours given off by intrusive igneous masses may contain substances which combine with the ingredients of the surrounding rocks and thus modify their composition. Boron, fluorine and phosphorus are the principal elements which are transferred in this way, and minerals such as tourmaline, topaz and mica are the characteristic products in quartzose or argillaceous rocks; while apatite, fluorspar, axinite, datolite and chondrodite are commonest in limestones. This is a form of pneumatolytic action (see ).

Extreme cases of the mutual interaction of the intrusive rock with the masses invaded by it are provided by the fragments enclosed in the molten magma (known as xenoliths). These are often rounded and eroded, as if softened or partly fused and dissolved. Similar changes are found in the rocks of the aureole for a few feet or yards where in actual contact with the granite. This belt of indurated hornfelses often weathers much more slowly than the igneous rock, and stands out as a prominent, sharp-edged ridge running round the granite margin.

Where sediments are dissolved in igneous rock we may expect to find modifications in the chemical composition and in the minerals produced on crystallization of the magma. Some granites, for example, which contain many rounded, partly dissolved enclosures of slate are themselves full of corundum, andalusite, cordierite and other minerals, which appear to indicate the effect of absorbed slate material. Much discussion has taken place as to the importance of such processes in modifying the facies presented by igneous rocks. Granites are alleged to have absorbed impure limestones and thus to be changed to diorites (Pyrénées). At the contact of the two rocks a narrow zone of diorite intervenes between the granite and the limestone. In this case an acid rock has become basic (or intermediate) in character; similarly, basic rocks—such as gabbros—are said to become granitic where they have melted down large quantities of felspathic quartzite. On the other side it is argued that as precisely the same modifications of the igneous rocks are known to occur where these explanations cannot possibly hold good—e.g. zones of diorite at the contact of granite with quartzite or mica-schist—they are really due to chemical segregation or differentiation in the magma and not to any admixture with foreign material.

Such modifications in the igneous rock at its contacts are often said to be endomorphic, while those which take place in the aureole or country rocks are exomorphic. The endomorphic changes are not always strictly of the nature of contact alteration. The commonest are the presence of a fine-grained, sometimes glassy, chilled edge due to rapid solidification from sudden cooling of the magma. The fine-grained marginal facies is often porphyritic, while the interior of the mass is granular or eugranitic. There is often a tendency to the development of special minerals in the edge of intrusive masses. Some of these arise probably from absorption of country rock, e.g. cordierite, andalusite, iron oxides (in granite). At the same time there may be a great abundance of angular or rounded enclosures, so that the marginal rock is brecciform. Where granite penetrates gabbro the fragments of the latter are sometimes melted down and digested in the granite till only the crystals of their augite or diallage are left (Skye). Granite margins are not always more basic than the average of the mass; they may be exceedingly rich in quartz and at the same time very coarse-grained or pegmatitic. This seems to arise from the production of fissures at the contact after the granite has to a large extent solidified. In these fissures the pegmatites are laid down by escaping vapours. Metasomatic changes are especially common also in this situation, and have often formed very valuable mineral deposits along igneous contacts. There also pneumatolytic processes often concentrate their attack; schorl-rock, greisen, topaz-rock and china-stone (or kaolinized granite) are characteristic products, and the active vapours often transform the sediments around, forming schorl-schist, calc-silicate rocks and sericite-schists.

Regional Metamorphism.—The second kind of metamorphism is known as “regional” because it is not confined to narrow areas like contact metamorphism, but affects wide tracts of country. Metamorphic rocks of this kind often cover a large part of a continent (e.g. the centre of Africa or Scandinavia and Finland). Whatever the causes be which produced it, they must have been of widespread operation and connected either with great geophysical processes or with definite stages of the earth's development. Where such rocks occur there is generally much evidence of earth movement accompanied by crushing and folding. They are very characteristic of the central axes of great mountain chains, especially when these have been denuded and their deeper cores exposed. Most geologists believe that this connexion is causal, holding that the contraction of the outer layers of the earth's crust, due to shrinkage of a nearly rigid shell upon a cooling and contracting interior, has bent and folded the rocks, and at the same time has crushed and largely recrystallized them. According to this view regional metamorphism is the result of pressure and folding; hence the name dynamo-metamorphism is frequently applied to it.

A great number of observations collected in all regions of the globe may be adduced in support of this hypothesis, forming a mass of evidence so strong as to be almost overwhelming. The structural features which prove that there has been great folding in these rocks are accompanied by microscopic and lithological characters which demonstrate that extensive crushing has taken place. Through progressive stages a slate with fossils may be traced into a phyllite, which becomes a mica-schist, or, in places, a micaceous gneiss. At first the fossils are distorted or torn apart, but they disappear as crystallization advances. Limestones under great pressure flow almost like plastic masses, losing their fossils and becoming crystalline. Grits, quartzites and granites show the effects of crushing in the pulverization of their minerals and the breaking down of their original clastic or, igneous textures, fine slabby s (q.v.) and granulites being produced. Moreover, the degree of metamorphism in the rock can often be shown to correspond closely to the extent to which it has been folded and crushed.

Another argument in favour of dynamo-metamorphism, which has been urged with much insistence by the extreme supporters of these theories, is the retention of original chemical characters in the metamorphic rocks. Some of them bear unmistakably the stamp of sedimentary origin, e.g. the limestones and marbles, quartzites, graphite-schists and aluminous mica-schists. Others have the normal composition of granites, diorites, gabbros and other types of plutonic igneous rocks. This leads to the inference that these were originally normal sediments and intrusives or lavas, and that their present crystalline state and foliated structure are the result of agencies which operated on them subsequently to their formation. Where the degree of metamorphism is not too high, and the folding and dislocation not too complex, the sandstones, shales and limestones may be mapped out, and igneous bosses, dikes and sills, with their contact aureoles, veins, pegmatites and segregations, convincingly delineated on the maps. This shows that a whole complex or terrane, consisting of diverse petrological types of normal sediments and igneous rocks, may be converted by metamorphism into a great series of gneisses and schists. Although recrystallization has been complete, the original rock masses still retain their identity in their new state.

The metamorphism in a rock series may be of nearly uniform intensity over a large area; the sediments, for example, may have all their clastic and organic structures effaced, and in the igneous rocks the porphyritic, ophitic, graphic and other textures may have completely disappeared. This, however, is not always the case, especially when the metamorphism is not of very intense degree. Parts of the rock may retain original structures, while others are typical crystalline schists and gneisses. Kernels, lumps or phacoids of massive rock are often found embedded in schists, and it is clear upon inspection that the phacoids represent the original state of the rock, while the schist is the effect of metamorphism. At other times a rock mass, such as an intrusive sill, is schistose at its edges and surrounded by schistose sediments, while near its centre it is almost entirely massive. The hard igneous rock has proved more rigid than the soft and plastic sediments; in folding, the latter have yielded to the stresses, and internal movement has produced foliation. The crystalline rock of the intrusive sheet has been strong enough to withstand the pressures and has folded like a rigid mass. At the junctions the effect of differential movement is shown by the presence of a belt of rock which often has a most pronounced schistosity. Some intrusive dikes show foliation especially marked along their edges; or they may be traversed by planes of movement, running obliquely or directly across them, and characterized by, the development of very marked schistosity. Exceedingly sudden transitions between normal igneous rocks and schists or gneisses have been described in sheared dikes. A normal dolerite, with ophitic structure and abundant augite, has been shown to pass in a few feet or inches into an epidiorite, where hornblende has replaced the primary augite, and lastly into a perfectly typical hornblende-schist, completely recrystallized with development of epidote, green hornblende, sphene and other minerals of metamorphic facies from the original constituents of the dolerite. These phenomena are regarded as establishing that the rock had consolidated as a normal dolerite before the processes which caused the metamorphism began to act; that these processes resulted in internal movement in the rock