Gneiss is defined as a metamorphic rock composed essentially of quartz, feldspar and a mica etc. The quartz and feldspar occur together in layers which are separated from each other by thin drawn-out bands of mica. A gneiss has usually a light color, although this is not necessarily so. Gneiss is a very common rock type, especially in regions in which the oldest rocks, those of the Achaean age, are found. The term gneiss is not only the name of a particular kind of metamorphic rock, but also, in a wider sense, it is used as an expression of a certain texture. Thus when we use gneiss as a name in the limited sense, we mean a rock which has the composition of granite quartz, feldspar, and mica with a certain foliated texture. Various varieties of gneiss have received distinctive names, most of which are self-explanatory, like banded-gneiss, lenticular gneiss, biotite-gneiss, hornblende-gneiss, granite-gneiss, diorite-gneiss, syenite-gneiss, With these specialized terms, we use the term gneiss in the wider sense, and denote rocks whose composition is indicated by the first word, and the texture by the second. The only general definition of gneiss which will cover all cases is that they are metamorphic rocks, composed of feldspar, with other minerals, which have a certain characteristic texture. But, as everywhere generally used when no qualifier is prefixed, common gneiss, which is composed of quartz, feldspar and mica, as stated above, is understood, and the term is so employed in this book. If the wider sense is meant the qualifier is given.
Mineral Composition. Various kinds of feldspar are found in gneisses, both the alkalic and soda-lime varieties, but they can rarely be distinguished by megascopic means. The mineral is white to gray in color, or reddish, as in granite, and is apt to be in more or less round, or elongated, lenticular, formless grains; this lack of definite form makes it more difficult to distinguish from the quartz than in most granites, and the cleavage should be carefully sought. Sometimes large grains, the size of a pea, or even larger, occur, giving the gneiss a porphyritic character; if the cleavages of these are examined against the light, it may be often observed that they are Carlsbad twins. Such large crystals may indeed have been the phenocrysts of a former porphyritic granite, or they may have been feldspar pebbles of a conglomerate or arkose, or they may have been made by injected material.
The quartz is also in more or less round grains or lenticular masses, or in granular aggregates with the feldspar. Its color is white or gray, sometimes reddish, rarely bluish. In the larger grains it is easily recognized by its greasy luster and conchoidal fracture. The mica may be either biotite or muscovite, or a mixture of both. The biotite is black or dark brown, the muscovite is white or yellowish to light brown, sometimes pale green. The mineral does not have any distinct crystal form, but is in flakes, shreds or irregular leaves, drawn out in bands, or in thin patches. It usually lies stretched out along the structure planes of the rock, and in large part its easy cleavage, thus arranged in one direction, conditions the schistosity or cleavage, and gives emphasis to the gneissoid texture. Thus the surface of chief fracture of a flake of the rock may appear to be largely coated with mica, and, judging from this alone, one would be apt to gain an exaggerated idea of the relative amount of it in the rock; the surface of cross fracture should also be examined to gauge correctly its relative amount, as compared with the other mineral constituents. This is also especially true, in the mica schists, and in those gneisses, which, by decrease of feldspar and increase in mica, form transitions into these latter rocks. This effect is also more marked in many gneisses, because there is a tendency for the quartz and feldspar to be collected in layers, which alternate with layers of mica. Hornblende may occur in gneisses, sometimes associated with the mica, sometimes alone, forming a special variety. It is seen in dark, prismatic crystals without good terminations, as in granite, syenite, etc. Minute crystals may be aggregated into flattened lumps and layers.
Besides these, many other minerals may occur in gneisses, sometimes so prominently as to form special varieties. Of these garnet, of a dark red common variety, is perhaps the most conspicuous. The crystals are sometimes large, as compared with the size of the other rock constituents. Epidote may also be discovered, as well as graphite, in some varieties. Sillimanite, a mineral with the same composition as andalusite and kyanite, is sometimes seen in gneiss, in bundles and brush-like groups of slender fibers or prisms. Tourmaline occurs also under circumstances similar to those which obtain in granite. In some gneisses the mica may be partly, or wholly, replaced by chlorite, usually from alteration. Texture. This has been already described in large part under the general remarks on metamorphic rocks and what has been said above respecting the mica. The essence of the texture consists in the layers of mingled quartz and feldspar, which are separated by drawn out layers of mica. Where the amount of mica is small, the gneissoid texture is less evident, and it increases with the increase of mica. Sometimes these layers are thick and coarse, giving a pronounced gneissoid effect, sometimes the layers are extremely thin. In some cases the layers continue their individual character for considerable distances, in others they are very short, lenticular, and are closely interlaminated. According to these appearances, different varieties of gneiss have been named on a textural basis. The gneissoid texture is sometimes scarcely perceptible in a hand specimen, but clearly seen on a large, exposed surface of the rock. This is especially the case in rocks which were originally granites, but which, by pressure and shearing, have been converted into gneiss.
The texture described above, the banding or schistosity, may extend for long distances in straight, regular lines, or it may be curved, folded, contorted, or faulted, often in the most complex and remarkable manner, and on any scale, even to a very minute or even microscopic one. Examples of such intricately folded and compressed gneisses testifies in general to repeated dynamic movements, with shearing and folding, the earlier ones producing the gneissoid structure and the later ones crumpling it up, though it is possible that in some cases the two things are simultaneous, In some gneisses, as in some granites, a definite porphyritic texture may be present, with large and definite crystals of feldspar, which show more or less distinct crystal form. Such gneisses are to be generally regarded as originally porphyritic granites, which have had the gneissoid texture imposed upon them, though in some cases, it may be, that the large crystals have been formed in gneiss of a different origin by growth from injected material. Such gneisses are allied to, and may pass over into, types, which, with a short, thick, lenticular texture, contain ovoid masses of feldspar or quartz. The ovoid bodies are called "eyes" (German, augen) and the rocks containing them "augen-gneiss'' from the German name, or "eyed-gneiss." As explained, the texture of metamorphic rocks, they may be of quite diverse origin. In some gneisses are to be seen pebbles of various kinds of previously existent rock masses, of granite, quartzite, etc. They are apt to be drawn out into flattened lenticular masses, but their original character is evident, and it is clear that the gneiss in such a case was originally a conglomerate, whose finer material has been metamorphosed, leaving the larger pebbles mostly unchanged, save in shape.
Color: The color of gneisses is too variable a feature to be of any value as a special character. It depends on the color of the quartz and feldspar, and on the relation of these to the amount of biotite, or other dark colored minerals, they may contain. Generally they are light colored, however, in gneisses of sedimentary origin, carbonaceous material may be present and in the form of graphitic material color the rock very dark. Hence we find them from almost white passing through light shades of red or gray into darker ones, into brown and green, and even black.
Chemical Composition: As the sources of the material from which gneisses have been made are varied, so do we find great variability in their chemical composition, so much so that this character cannot be relied upon as having any special value as one of their definite features. Since they are composed of quartz and feldspar in notable amount they must contain silica, alumina and alkalies, and they usually have also more or less iron and lime, but these oxides may vary within wide bounds. Of the mineral varieties, by common gneiss, or "gneiss" for short, mica-gneiss is meant. If further distinction is required, the kind of mica present may be stated, and we thus have biotite-gneiss, muscovite-gneiss, or biotite-muscovite-gneiss. If the mica is accompanied or replaced by some prominent mineral, as is often the case, other varieties are formed, such as hornblende-gneiss, epidote-gneiss, tourmaline-gneiss, garnet-biotite-gneiss, etc. The different prominent minerals, which may thus take part, have been already described under composition. Of the varieties based on general composition, it may be said that all of the different varieties of coarser-grained, feldspathic, igneous rocks may occur with pronounced gneissoid texture. In accordance with this we have granite-gneiss by far the most common variety syenite-gneiss, diorite-gneiss, and even gabbro- and anorthosite-gneisses. Sometimes this texture has been imposed upon the igneous rocks after they had solidified, by intense pressure and shearing, and sometimes while they were still soft, pasty and crystallizing, by forced differential flowage, due to various causes.
Inclusions in Gneiss: It is very common to find inclusions, or smaller rock masses, embedded in gneiss, which differ in a marked degree in mineral composition, texture, etc., from the main rock body which encloses them. Thus lenticular masses of quartz frequently occur, and of very variable size. They may be the remains of a quartz pebble of a conglomerate, as explained under texture, or they may have been deposited from solution in some lenticular cavity, opened in the folding of the rock masses. This case may sometimes be detected, in that the quartz mass tends to possess a comb structure, being composed of an aggregate of crystals whose prism directions are set perpendicular to the wall of the cavity. In many gneisses irregular spots, streaks, and lines of pegmatite occur, similar to those in granite. In addition to the quartz, feldspar and mica, they often contain the accessory minerals seen in granite-pegmatite dikes, such as tourmaline, apatite, beryl, garnet, topaz, etc. In the latter case they probably represent the remains of former granite-pegmatite dikes, which have been folded up or squeezed out in dynamic metamorphic processes, but not all of the pegmatitic modifications seen in gneiss are to be certainly ascribed to such an origin, for they may have been produced by secretions from later solutions of heated waters moving through the rock mass. The beautiful crystals of orthoclase, of the varieties called adular and moonstone, occurring in some gneisses, have been probably produced in this way.
Also there are frequently seen in gneiss, spots, streaks and irregularly curved and winding ribbons of white or pink felsite, or fine-grained granite similar to the aplite of granites. These may be former aplite dikes folded up, or later granitic intrusions or secretions from heated solutions. They are sometimes seen in the most complicated systems of network passing through the rock, and they may not have any definite wall against the gneiss, as is the case with regular aplite dikes. By their foldings, faultings, and contortions, they often show very clearly the movements which the general rock body has undergone. Included masses of other kinds are also frequently met with in gneiss. Thus the streaks and smears, produced by aggregates of the dark-colored or ferromagnesian minerals, such as are seen in granites and are described as "schlieren," are found in gneiss, and may have a similar origin. Also, irregular masses, strips, and lenticular bodies of other schists occur, which, if the gneiss has been derived from a former mass of igneous rock, may have been included or enveloped fragments of the stratified beds, into which it was intruded.
General Properties and Uses of Gneiss: Those gneisses, which under the action of metamorphic agencies have been thoroughly recrystallized, form solid and massive rocks, whose general properties closely resemble the massive igneous ones. Thus granitic gneiss closely resembles granite, and is used in the same manner for building and structural purposes. But often gneiss contains so much mica, that it has too easy a cleavage to be of much value. In general a gneiss should be so placed, that the plane of chief fracture lies in the mortar bed with the cross fracture exposed; otherwise it is liable, like some sedimentary rocks, to split and scale badly. Those gneisses which have assumed their texture under conditions of dry crushing and shearing are very tender and friable rocks, which fall to pieces readily under the blow of the hammer, and are of little value. The granite-gneiss of portions of the Alps, and the anorthosite-gneiss of parts of the Adirondacks, are examples of this. The jointing, erosion forms, etc., of granite-gneiss are similar, in general, to what is stated under granite. So also is the weathering, and gneisses form fertile sandy soils, which pass into loamy ones, as the decay of the feldspar and its alteration into kaolin becomes more complete.
Occurrence of Gneiss: Gneiss, especially common or mica-gneiss, is one of the most common and widely distributed of rocks. The occurrence of the metamorphic rocks in general has been already commented on, and it was stated that they are found in mountain regions and in those areas where the sedimentary beds have been eroded, as a basement upon which these later rocks rest. In such places common gneiss is usually the most prominent rock. Owing to this, it is spoken of by many geologists as "the basal gneiss," or "fundamental gneiss," and as, in many places, it is clearly the oldest rock of which we have any knowledge, some believe that they see in it the primitive crust of the earth. The Archaean, as it is now used as a division of geologic time, is almost entirely composed of gneiss, and to attempt to mention all the localities of the rock, would be practically equivalent to a description of the occurrence of the Archaean. Gneisses are not of course restricted to the Archaean; they occur in later formations, into the Mesozoic. Gneisses are found all over New England, and southward along the Piedmont plateau into Georgia; in the Adirondacks; in the Rocky Mountains' region, the Sierra Nevada in California, and other places in the United States; they cover large parts of eastern Canada and are prominent in Scotland, Norway and Sweden, Finland, parts of Germany, and in the Alps. In all of these regions different varieties, such as hornblende-gneiss, occur associated with the common kind.
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