Gneiss

“ The gneiss at Knock is exceedingly various in its composition, and many of its strata the geologist would fail to recognize as gneiss at all. We find along the precipices its two unequivocal varieties, the schistose and the granitic, passing not unfrequently, the former into a true mica schist, the latter into a pale feldspathose rock, thickly pervaded by needle-like crystals of tremolite, that, from the style of the grouping, and the contrast existing between the dark green of the enclosed mineral, and the pale flesh-color of the ground, frequently furnishes specimens of great beauty. ”

“ Thus there is in the older formations a genealogy of rocks which, in the absence of fossils, may be used with some confidence, but which does not apply to the more modern deposits. Still, nothing in geology absolutely perishes or is altogether discontinued; and it is probable that, down to the present day, the causes which produced the old Laurentian gneiss may still operate in limited localities. Then, however, they were general, not exceptional. It is further to be observed that the term "gneiss" is sometimes of wide and even loose application. ”

“ I have found the same stone existing, in a decomposed state, as a very tenacious clay, among the gneiss strata of the hill of Cromarty; and oftener than once had I amused myself in fashioning it, with tolerable success, into such rude pieces of pottery as are sometimes found in old sepulchral tumuli. Such are a few of the rocks included in the general gneiss deposit of Sleat. ”

“ Approaching the point of Sleat the schist gradually becomes green and glossy, thus passing into chlorite slate, and here we arrive by an insensible gradation to the variety of gneiss which I formerly described as found there. In thus pursuing the red sandstone on the western side of Sleat, the reason appears why I could not when speaking of that rock, define the boundary of the gneiss on this side of the island, and I need now scarcely repeat, that any attempt to examine rocks so constituted, in the interior country, would leave nothing but doubt and uncertainty ”

“ A [31] large part of the material used for building purposes, under the name of granite, is obscurely marked gneiss. In all primary countries it is an abundant rock, occupying extensive districts, and sometimes forming mountain masses.
2. Mica slate lies next above gneiss, and is a very abundant rock. As it differs from gneiss only in the proportion of mica which it contains, and as the quantity of mica in it is very different in different places, it is often difficult to make the distinction between them. It also passes by insensible degrees into the argillaceous rocks. ”

“ To begin with the oldest set of rocks, No. 1; they consist chiefly of hornblendic gneiss, and in the neighbouring Hebrides form whole islands, attaining a thickness of thousands of feet, although they have suffered such contortions and denudation that they seldom rise more than a few hundred feet above the sea-level. In discordant stratification upon the edges of this gneiss reposes No. 2, a group of conglomerate and purple sandstone referable to the Cambrian (or Longmynd) formation, which can elsewhere be shown to be characterised by its peculiar organic remains. ”

“ The rocks to be joined are of the two types represented in Fig. 3, p. 106 (for convenience' sake I shall in the rest of this chapter call the slaty rock gneiss, and the compact rock protogine, its usual 206 French name) . Fig. 55 shows the general manner of junction, beds of gneiss occurring in the middle of the protogine, and of protogine in the gneiss; sometimes one touching the other so closely, that a hammer-stroke breaks off a piece of both; sometimes one passing into the other by a gradual change, like the zones of a rainbow ”

“ This region, nearly coincident with the mountainous upper portion of the island, is chiefly composed of the following crystalline rocks: gneiss, which is the most common of them all, quartzite and quartz-schist, with occasional beds of crystalline limestone and mica-schist, although this latter rock is very rare. The gneiss is mostly grey, but occasionally pinkish, its essential constituents (felspar and quartz) being almost always associated with dark mica (biotite) and hornblende in variable quantity. The rock is therefore a hornblende-granitite-gneiss. ”

“ In the small island of Oransa, and still more conspicuously in an islet adjoining to it, there is to be found a mass of actinolite rock, which can also be traced to the adjoining shore of Sky near to Camiscross. It lies among the gneiss, holding an uniform and parallel course with it, and as the beds of gneiss are here nearly vertical, it presents the appearance of a vein, its edge alone being visible. Nevertheless, its conformity with the gneiss, the analogy which it bears to common hornblende-schist, and its actual gradation into that substance, leave no doubt respecting its true character. ”

“ The interior of the country is too much encumbered with peat and with vegetation to permit of any decision on a subject so obscure as is the point of change between the gneiss series and the rocks which follow it, and I must therefore limit myself to the appearances which occur on the sea shore, where every change can be traced in the most minute manner. Here there will be found a transition, unexpected and improbable as it may at first seem, between the two series, that of the gneiss, and that of the graywacké and sandstone which follows it ”

“ Down by the river the walls are composed of black gneiss, slates, and schists, all greatly implicated and traversed by dikes of granite. Let this formation be called the black gneiss. It is usually about 800 feet in thickness.
Then over the black gneiss are found 800 feet of quartzites, usually in very thin beds of many colors, but exceedingly hard, and ringing under the hammer like phonolite. These beds are dipping and unconformable with the rocks above; while they make but 800 feet of the wall or less, they have a geological thickness of 12,000 feet. ”

“ On the road we examined various pieces of granite and of gneiss, which with all their differences seem, nevertheless, to have a common origin. At last we stood before the waterfall, which well deserves its fame above all others. At a considerable height a strong stream bursts from a cleft in the rock, falling downward into a basin, over which the foam and spray is carried far and wide by the wind. The sun at this moment came forth from the clouds, and made the sight doubly vivid. ”

“ When we have a considerable thickness of metamorphic rocks which still exhibit more or less distinct traces of bedding, like the successive beds of gneiss, mica-schist, and quartz rock of the Scottish Highlands, we can hardly doubt that the now crystalline masses are merely highly altered aqueous strata. But there are cases where even the bedding becomes obliterated, and it is then much more difficult to determine the origin of the rocks. Thus, we find bedded gneiss passes often, by insensible gradations, into true amorphous granite. ”

“ A further characteristic of this Hercynian gneiss is the frequent intercalation of beds of rocks rich in hornblende, such as hornblende-schist, amphibolite, diorite, syenite, and syenitic granite, and also of serpentine and granulite. Beds of granular limestone, or of calcareous schists are also never altogether wanting; while iron pyrites and graphite, in lenticular masses, or in local beds conformable to the great mass of the gneiss strata, are very generally present. ”

“ Only where these mighty gneiss ranges break the monotony does anyone examining the country with an eye to its economic value find full satisfaction. Gneiss weathers easily and forms excellent soil, as the natives have long ago discovered; and, though they by no means despise the less fertile tracts, yet the most favoured sites for settlements have always been those in the immediate vicinity of the gneiss islands. Masasi, with its enormous extent, taking many hours to traverse, is the typical example of such economic insight. ”

“ Other gneisses are igneous (orthogneisses) . These have very much the same composition as acid igneous rocks such as granite, aplite, hornblende granite, or intermediate rocks such as syenite and quartz diorite. Many of these orthogneisses are not equally well foliated throughout, but are massive or granitoid in places. They are sometimes subdivided into granite gneiss, diorite gneiss, syenite gneiss and so on. The sedimentary schists into which these rocks have been intruded may show contact alteration by the development of such minerals as cordierite, andalusite and sillimanite. ”

“ Igneous Rock.—Granite, as was stated before, is found almost universally beneath gneiss and mica schist,— sometimes touching one (gneiss most frequently) , some. times the other. It generally appears to have been in a state of fusion since the deposition of these superincumbent strata, since veins of it are injected into their cracks and fissures. (Examples may be seen in Glen Tilt, in Arran, in Skiddaw, in Wicklow, &c.) ”

“ It is a favourite speculation among a certain class of modern geologists, that the peculiar mineral, and structural characters of gneiss and mica schist are not original, but derived from the influence of heat upon common sandstones and shales—a greater effect of which heat would convert the gneiss to granite; and they suppose that such transformation of the substance of the rocks was accompanied by a complete extinction of the substance and impressions of the imbedded organic fossils. ”

“ The felspars of the gneisses are predominantly orthoclase (often perthitic) , but microcline is common in the more acid types and oligoclase occurs also very frequently, especially in certain sedimentary gneisses, while more basic varieties of plagioclase are rare. Quartz is very seldom absent and may be blue or milky and opalescent. Muscovite and biotite may both occur in the same rock; in other cases only one of them is present. The commonest and most important types of gneiss are the mica-gneisses. Hornblende is green, rarely brownish ”

“ The ultimate foundation of the country above the gháts, [15] as of most of peninsular India, is gneiss. Various other kinds of rock of less but varying antiquity have been superimposed upon different parts of the district. The gneiss is usually uppermost throughout Bhadráchalam, Chódavaram and the eastern portion of Pólavaram, and, in the form of what is called Bezwáda gneiss, throughout Yellavaram and much of Tuni as well as in the north of Rajahmundry and Peddápuram taluks. ”

“ The bands may show color differences, too. For example, a pink layer made up of feldspar grains may be found next to a black layer made up of hornblende grains. The mineral grains interlock as they do in igneous rocks, and they are generally large enough to be seen without a magnifying glass.
Gneiss from Blanco County, Texas, showing light and dark bands.
Gneiss can form from an igneous rock, such as granite, or from a sedimentary rock, such as sandstone. Heat, fluids, and pressures below the earth’s surface change these rocks into gneiss. ”

“ I have been thus particular in describing this singular rock, because it has often been confounded with gneiss, from which it is strongly distinguished in its mineral character, and because it is important that a substance of so much consequence in the system of rocks as this is should have its relations accurately ascertained, and should not from laxity of definition be introduced into a series with which it has no geological connection. Many of the Western islands offer examples of it in its most genuine form. ”

“ Professor Keilhau drew my attention to several localities in the country near Christiania, where the mineral character of gneiss appears to have been affected by a granite of much newer origin, for some distance from the point of contact. ”

“ Laurentian gneiss forms the greatest mass of the exposed rocks of the country bare of ice. They are found on both sides of Smith Sound, rising to heights of 2000 ft., and underlie the Miocene and Cretaceous rocks of Disco Island, Noursoak Peninsula and the Oolites of Pendulum Island in East Greenland. Ancient schists occur on the east coast south of Angmagssalik, and basalts and schists are found in Scoresby Fjord. ”

“ It is possible to reconcile them only by supposing that the red sandstone series belongs to the primary rocks, and the whole of these strata which extend from the point of Sleat to the limestone of Strath, will thus form a succession of rocks alternating with and graduating into each other. That the gneiss is a primary rock can, I imagine, admit of no dispute. ”

“ There is a variety of rock—a crystallized rock—with bands all through it, called gneiss (say "nice") . Gneiss is made from all kinds of rock including, of course, conglomerate; that is to say "pudding stone" [16] warmed over.
[16] "Pudding stone" is a rock with pebbles all through it, like the plums in a Christmas pudding. Its book name is "conglomerate." ”

“ As a matter of fact, a gneiss may exist in which there is no differentiation into layers; the whole of the constituents have been drawn out and elongated, any mica present becoming naturally conspicuous by its - 156 - flattened wisp-like forms. The banded gneisses, on the other hand, where layer-structure is obvious, consist in reality of bands of different rock-types. Sometimes all the layers are granitoid, but one band will contain only quartz and felspar, while another will contain the same minerals with an admixture, and perhaps a great predominance, of mica. ”