invalidate the general rule. Thin bands of limestone, for
example, may be followed for miles in belts of mica-schist
or gneiss, never losing their identity by blending with the rocks
on either side of them. By tracing out zones such as these
it is often possible to unravel the highly complicated stratigraphy
of metamorphic regions where the rocks have been greatly
folded and displaced. Another important consequence of
the persistence of the chemical individuality of metamorphosed
rocks is that very often an analysis indicates in the clearest
possible fashion what was the original nature of the rock mass.
Sandstones, limestones, ironstones, shales, granites, dolerites
and serpentines may be totally changed in structure and very
completely also in mineral composition, but their chemical
characters are practically indelible. Confusion arises sometimes
from the fact that two rocks of different origin may have much
the same composition, e.g. a felspathic sandstone may closely
approach a granite, or an impure dolomite may simulate
a basic igneous rock. Individual specimens, consequently,
cannot always be relegated with perfect certainty to sediments
or igneous rocks; but in dealing with a complex containing
a variety of types the geologist is rarely long in doubt as to
their original nature.
Two distinct kinds of metamorphism are recognized, namely contact or thermal metamorphism, and folding or regional metamorphism. The former is associated with intrusive masses of molten igneous rock which were injected at a very high temperature and produced extensive changes in the surrounding rocks. The second occurs in districts where earth folding and the movements attendant on the formation of mountain ranges have flexured and crushed the strata, probably at the same time considerably raising their temperature. Although these processes are very different in their origin, and in the great majority of cases produce quite different effects on the rocks they involve, there are instances in which the results are closely comparable. A sandstone may be converted into quartzite and a limestone into marble by either kind of metamorphism. It is best, however, to describe them as phenomena essentially different from one another.
Contact Metamorphism (thermo-metamorphism).—Any kind of rock—igneous or sedimentary—which has come in contact with an igneous molten magma is likely to show alteration of this type. The extent and intensity of the changes depend principally on two factors: (1) the nature of the rock concerned, and (2) the magnitude of the igneous mass. It is to be expected that a great intrusion of granite will produce more extensive effects of this kind than a narrow dike a few inches or a few feet broad. At the edges of such dikes only a slight induration may be noticeable in the country rock, or there may be recrystallization with formation of new minerals for a few inches. Rarely does the alteration extend beyond this. Shales are baked and hardened, sandstones are rendered more compact or occasionally are partly fused, limestones may be converted into marble containing garnet, wollastonite, augite or other calc-silicates. A great granite boss, which may be ten or twenty miles broad, is often surrounded by a wide aureole of contact alteration. This may be a few hundred yards broad or a couple of miles; in rare cases the breadth of the aureole is only a few yards. These variations may have structural causes; thus when the aureole is narrow the junction of granite with country rock may be vertical; when the aureole is broad the granite may be a flat-topped mass which dips at low angles outwards on each side. When a broad aureole accompanies a vertical junction we may suppose that molten rock has flowed upwards along this boundary line for a prolonged period, and has gradually raised the rocks to a very high temperature, even at some distance away from the contact. Where the alteration is slight and local there is usually something in the composition of the rocks or in their crystalline state to account for this.
No less important is the nature of the rocks involved. Where a granite intrudes into a succession of various types of sedimentary and igneous rocks the differences in their behaviour are often very marked. Sandstones alter less readily than shales or slates, and limestones, especially if they be marly or argillaceous, are often full of new minerals, when purer shales on each side of them are not visibly affected. Schists and gneisses, being already highly crystalline, are very resistant to thermal alteration, and may show it only for a few inches where they are in actual contact with the granite, or in minute fragments which have been broken off and surrounded by the invading magma. Igneous rocks, since they consist of minerals which have formed at very high temperatures, may show no change whatever. If they are decomposed, however, their secondary products, including those which fill veins and amygdaloidal cavities, are often entirely recrystallized in new combinations Instances of this will be given later.
The intensity of the alteration depends very greatly on the proximity to the intrusive rock. A typical aureole surrounding a granite boss, for example, consists of rocks in all stages of alteration, the most affected being nearest the granite, while as we travel outwards we pass over zones of successively diminishing metamorphism. Around the granites of Cornwall, the Lake District and Ireland there are tracts of altered slate which show these stages very well. The first sign of metamorphism is a slight increase in hardness and glossiness, making the slate a little brighter and more brittle. This is due to the formation of mica in small crystalline plates mostly parallel to the cleavage of the rock. Nearer the granite a faint spotting is visible on broken surfaces of the slates, and this becomes more pronounced as we enter the middle part of the aureole. These spotted slates, in Cornwall for instance, often occupy a zone a mile in breadth. They are less fissile than the unaltered slates and have rounded or elliptical spots about a quarter of an inch across. The spots are usually darker than the body of the slate, though sometimes paler. Angular, branched, lenticular and rhomboidal spots sometimes occur. Under the microscope these rocks consist mainly of brown mica, quartz and organic matters, iron oxides, &c.; the spots may be due to aggregation of biotite or of quartz, but often differ little in composition from the surrounding rock. Their dark colour is due to, abundance of iron oxides or graphite, with chlorite and biotite. Still closer to the granite a development of crystals takes place in the slates; the commonest are andalusite, chiastolite (with cross-shaped dark enclosures), cordierite, staurolite and garnet. At the same time the minerals formerly enumerated crystallize in larger individuals (biotite, quartz, iron oxides, &c.), so that the rock becomes rather more coarse-grained. At this stage the fissility and cleavage structures of the slate tend to be obliterated, and the rocks are dark, lustrous (from the abundance of mica), hard and splintery. To this type the name hornfels is given. The innermost zones of the aureole consist mainly of hornfelses, and where there are slate fragments enclosed in the granite they usually show these characters in their most pronounced form.
The nature of the new minerals produced depends principally, of course, on the chemical composition of the rocks affected? In pure sandstones only quartz is formed, and pure limestones merely recrystallize as marbles. Argillaceous rocks are characterized by abundance of alumina; hence, when thermally altered, they may contain corundum, or silicates of alumina such as sillimanite, kyanite, andalusite and chiastolite. Most rock masses, however, are far from pure and hence the variety of minerals which may arise in them from contact alteration is very great. Argillaceous limestones, for example, very frequently contain garnet, vesuvianite, wollastonite, diopside, tremolite, sphene, epidote and feldspar; that is to say, minerals in which lime is present along with silica, alumina, magnesia and other substances. Calcareous sandstones yield augite, garnet, sphene, epidote; argillaceous sandstones are characterized rather by biotite, sillimanite and spinel.
In each case the materials already present in the rock have united to form new mineral combinations. Crystallization has been stimulated by the rise of temperature, aided, no doubt, by moisture. Water vapour, even at comparatively low temperatures when the pressure is considerable, is a powerful mineralizing agent and greatly facilitates crystallization. Often the rocks acquire) ultimately a pseudoporphyritic or porphyro-blastic structure, as they contain large or conspicuous crystals scattered through a finer grained ground-mass; not only these porphyritic ingredients but the body of the rock shows increased crystallization, for contact alteration as a rule makes rocks more coarse-grained than before.
In rare instances fusion may take place, but this must be exceptional, as the finest original structures are often very perfectly preserved by rocks which have been in great measure recrystallized. Finely laminated argillaceous sandstones, for example, may pass into cordierite—or andalusite—hornfelses showing a mineral banding which corresponds exactly with the original lamination. For this reason the newly developed minerals are not frequently of good crystalline form. When weathered out of the rock they have mostly rough, imperfect faces, but exceptions to this occur in garnet, staurolite, tourmaline and a few others which often produce good crystals even in these adverse circumstances.
is only true in a general way that the rocks which are thermally altered experience no change in their chemical composition. The new minerals which are substituted for the original ones are such as are stable at high temperatures. Many of the silicates which form a large part of sedimentary rocks contain combined water; examples are chlorite, kaolin and clay. The water, or part of it, is expelled, forming silicates with little or no water, e.g. biotite, felspar, andalusite. Carbonic acid may be retained or driven out; in a siliceous limestone the silica tends to combine with the lime producing calc-silicates by replacing the carbonic acid. In a pure limestone the carbonate merely recrystallizes as marble. This loss of volatile ingredients must occasion a diminution in the bulk of the sedimentary mass involved; in cooling there will be contraction, and
fissures are produced which may be filled with igneous dikes or with
