Mica schist

“ Chlorite schist.—A greenish, fine-grained metamorphic rock in which chlorite is the principal mineral, but in which magnetite is a quite characteristic accessory constituent.
[466]
Staurolitic garnetiferous mica schist.—A mica schist in which garnet and staurolite are so abundant as to be essential constituents.
Clay slate.—A metamorphosed mud stone or shale. In the process of metamorphism the rock has been hardened, given a slaty cleavage, and innumerable minute scales of mica have developed to produce a silky luster upon the cleavage faces. The color may be gray, green, purple, or black. ”

“ In the mica-schists of this group biotite or muscovite may be the principal mineral and often both are present in varying proportions; the mica has developed from the argillaceous matter of the original rock; in addition there is always quartz and sometimes telspar (albite or oligoclase) . A large number of minerals may occur as accessories, e.g. garnet, tourmaline, staurolite, andalusite, actinolite, chloritoid or ottrelite, epidote, haematite, and if any of these is abundant its presence may be indicated by the name given the rock, e.g. staurolite-mica-schist. ”

“ It has glassy feldspar, and in places much mica. Four miles west of Ballena the granite is filled with pegmatitic veins, running in a northwesterly direction. They carry brittle crystals of black tourmaline, garnet, and muscovite mica. East of Ballena the rock becomes coarser and somewhat gneissose, with an abundance of mica. There are no bedding planes, simply a parallelism of constituents. This schistose structure has a northwest direction. The gneiss changes to mica schist on the grade above Santa Ysabel. The schist alternates with occasional strata of quartzite; strike north 25° west. ”

“ When we now remember that the water-formed rock (the mica-schist) is standing on edge, a suspicion arises that the granite is a fire-produced rock, and has been the agent of this elevation, and the large wall of granite may have been intruded in a molten state between the beds of mica-schist. If it be objected that the granite vein is merely a portion of the strata of mica-schist, and was like them deposited from water, an inspection will dissipate this illusion ”

“ Mica-schists, composed mainly of golden-brown lustrous laminæ of biotite with more or less quartz, occur near the base of Gebel Zabara and at Gebel Sikait [10,626] , as well as in small quantity at one or two other points. They are always associated with gneiss, and appear to form irregular bands, alternating and mixed with talc and other schists. The laminæ of mica can seldom be separated in any large size, breaking up at a touch into small scales; they are often highly contorted. ”

“ They found nothing but a snow-bunting or two, too small to shoot except in extremity, and a sort of gray Arctic hawk, which promised to be but poor eating. Probably there would be ptarmigans back farther, but they did not see any. At the head of the inlet they found a brawling stream which descended from the hills over mica-schist ledges and along sands that sparkled with yellow mica. Harry sighted this mica as he stooped to drink from the stream, and scooped up a handful of it with eagerness. He called to Joe, and both examined it closely, but it was plainly mica. ”

“ The more basic portions have suffered a considerable amount of alteration; the secondary constituents, chlorite and epidote, become abundant, and occasionally the hornblende has been completely replaced by the latter mineral; the rock then becomes an epidote schist of a pale-yellowish colour. In Swinyards Hill there is a micaceous gneiss with garnets, and in Raggedstone Hill there are interesting varieties of contorted mica-schist containing a large proportion of quartz. Dykes of intrusive rocks occur in nearly all the hills, but become far more numerous towards the north ”

“ Like all metamorphic rocks, mica-schists are principally found in Archean areas; the great majority of them are of pre-Cambrian age. There are, however, in the Alps, Himalayas, &c., many rocks of this sort which are believed to be secondary or even tertiary; the evidence for this is not in all cases satisfactory, as of course the fossils, which if preserved would be sufficient to prove it, are nearly always destroyed by the metamorphism. Mica-schists are rarely of economic value, being too fissile for building-stones and too brittle for roofing-slates. ”

“ In this way they become schistose, and from their felspar minute plates of sericitic white mica are developed, giving the rock in some cases very much of the appearance of mica-schists. If there have been no phenocrysts in the original rock, very perfect mica-schists may be produced, which can hardly be distinguished from sedimentary schists, though chemically somewhat different on account of the larger amounts of alkalis which igneous rocks contain. When phenocrysts were present they often remain, though rounded and dragged apart while the matrix flows around them. ”

“ On the summit of the ridge, which may be about 4,500 feet above the sea, the rock is a grey schist, often shaly in texture, with the strike about east and west, and clipping at a high angle approaching the vertical. These beds may perhaps be identical with the schists, sometimes containing mica, and sometimes more calcareous in composition, which we afterwards found at the head of the Amsmiz valley, and with the rock, described as micaceous schist, seen by Washington in his ascent from Tasseremout. ”

“ Soc. Trans.") and Mr. Darwin ("Geological Observations in South America") on these obscure subjects. I may add that wherever I met with the gneiss, mica, schists, and slates, in Sikkim, very near one another, I invariably found that their cleavage and foliation were conformable. This, for example, may be seen in the bed of the great Rungeet, below Dorjiling, where the slates overlie mica schists, and where the latter contain beds of conglomerate. ”

“ In the phyllites also white mica (muscovite or sericite) is more abundant as a rule than in slate, and its crystalline plates are larger; the abundance of mica gives these rocks a glossy sheen on the smooth planes of f1ssility. Many of the best Welsh slates are rich in small scales of white mica, which polarize brightly between crossed nicols. The Cornish slates are still more micaceous and rather coarser grained, so that they might be called mica-slates or even phyllites. ”

“ In the ejected blocks of crystalline limestone of Monte Somma, Vesuvius, the most perfectly developed crystals of biotite (figs. 1, 2) , or indeed of any of the micas, are found in abundance, associated with brilliant crystals of augite, olivine, humite, &c.
Although biotite (black mica) is much more common and widely distributed than white mica, yet it is of far less economic importance. The small size of the sheets, their dark colour and want of transparency render the material of little value. ”

“ The plane of the optic axes may be either perpendicular or parallel to the plane of symmetry of the crystal, and according to its position two classes of mica are distinguished. To the first class, with the optic axial plane perpendicular to the plane of symmetry, belong muscovite, lepidolite, paragonite, and a rare variety of biotite called anomite; the second class includes zinnwaldite, phlogopite, lepidomelane and most biotites. Dark coloured micas are strongly pleochroic. ”

“ A granulated, schistose aggregate, chiefly of quartz and mica. Though the constituents and the form of the rock correspond very nearly with mica slate, it cannot be considered as the primitive mica slate rock. It is so slightly compacted that it can scarcely be kept from falling to pieces. Its position is nearly horizontal.
480 feet. A stratum of coal, half an inch thick. This stratum may be traced, at different intervals, one hundred and eighty feet along the drift towards its mouth. It lies between the strata of the last described schistose aggregate. ”

“ 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. ”

“ In many parts innumerable detached fragments may be seen imbedded in the trap, the whole of the schist displaying a greater disorder and confusion than the sandstone does in any of the instances hitherto described. In a few places another singularity may be remarked. Veins of different dimensions, and ramifying in different directions, are to be found traversing considerable portions of the rock, and in some instances terminating in the schist, with which they are continuous. ”

“ I think "Phil's Fairies" is just beautiful.
I went to the mountains in North Carolina last summer, and mamma and I went to a mica mine and got some specimens. They are very beautiful. I heard the explosion, and I saw the mica and rock come up in a barrel, which was hoisted by a windlass worked by a donkey.
Besides the thin transparent pieces of mica, we gathered some specimens of white and green rock with little shreds of mica glistening in them. They are lovely. I am seven years old.
Claire F. H.
I find Young People so interesting that I can not do without it. ”

“ Crystals of the hornblende may be observed shooting far into the body of the schist, so as to render it often difficult to assign the limits of each rock. In a less degree the quartz and felspar exhibit the same appearance. Together with this, the schist is singularly contorted, being bent, broken, and intermingled in a most confused manner with the rock that traverses it, while distinct detached fragments are often involved in the mass of granite. ”

“ Occasionally, portions of the siliceous schist are attached to it, the separation being marked by well defined planes, and, from the contrast of colour, very conspicuous. If there were any doubt that this chert was originally a portion of the lias indurated by the same process that has converted the shale into siliceous schist, it would be removed by the fact that on the western shore of this district. the two substances are found in situ, associated in the same manner and in various states of transition from common lias and shale to chert and siliceous schist. ”

“ Yet—in her heart—she knew that, though others might fall at the touch of that fearful scythe, she, Split Madigan, as fleet of limb as a coyote and as sound of heart as a young pine-cone, could never, never die; that the world could never be when her quick red blood should be quiet and her mountain-bred lungs should be stilled.
With a bound Sissy pushed the twins out of the door. She was at the bedside when Miss Madigan entered.
"Go outside, Sissy!" she commanded. "Can't you see you're exciting her? Isn't it hard enough for me to take care of her when she's so cross? She's not to be excited. ”

“ It then assumes an appearance exactly resembling that of hornstone or compact felspar, having a smooth texture, with a thin-edged fracture intermediate between the splintery and flat conchoidal. I may as well add here that the schist is often of an argillaceous, but highly indurated, character, and also that it often passes into hornblende slate, and this (it is worthy of notice) the more frequently as it approximates nearer to the granite. ”

“ The silence of the room was unbroken save for the little crisp sound as Micky turned the paper; then the letter fluttered to the rug at his feet and lay there, half-curled up, as if it were ashamed of the words it bore and wished to hide them.
Micky raised his eyes and looked at his reflection in the glass above the mantelshelf. The pallor of his face surprised him, and the look of passionate anger in his eyes.
He was a man of the world. ”

“ As he took his place at the head, his wrathful eye swept from Frances in her high chair, up along the line, past the twins, through Cecilia, Irene, and Kate, till it lighted upon Miss Madigan's good-humored, placid face. His sister's placidity was an ever-present offense to the father of the Madigans,—the most irascible of unsuccessful men,—and the snort with which he finished the inspection and took up the carving-knife had become a classic in Madigan annals long before Sissy brought down the house at the age of eight by imitating it one evening in his absence. ”

“ Isn't it always so with natural causes? The only one that one does not divine is always the simplest. Ah! if Mila knew what danger she is playing with, and what pain she might spare me by telling me the truth! I will press her so to-morrow that she will tell me everything!"
While Michel reflected thus, the crystal water murmured in its narrow basin, wherein quivered the spectre of the moon. ”

“ This was better, and in a sense it was also worse; for Sissy could plainly see La Tourtillotte, a gaunt, businesslike creature in short rainy-day skirt and sweater, her long, thin arms going like pump-handles, her dark, tense face set upon a goal which seemed ever to flee before her as her weary feet carried her slowly and still more slowly around the circular track.
Despite her shocked sense of propriety,—and the lawless young Madigans had very strict ideas as to the conventions for adults,—the ardor of the struggle, the uncertainty of the issue, seized upon Sissy. ”