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54 REPORT OF THE VERMONT STATE GEOLOGIST. present. Apatite, magnetite, titanite, tourmaline, and epidote are the remaining accessories. Calcite, part of the muscovite, and chlorite are of secondary origin. The microcline of this rock commonly shows the delicate polvsvnthetjc twinning which generally characterizes the rnineral. Its substance is usually quite fresh and glassy, but it is at times slightly altered and clouded at the center. Minute lenses and spindle shaped bodies of plagioclase are intergrown with the microcline. Their index of refraction is higher than that of their host, and from the extinction angles they are believed to he oligoclase. The quartz is perfectly clear and glassy as a rule, but at times the mineral is clouded over with gas inclusions and cavities. containing bubbles. A rude approximation to crystal outlines against anhedra of microcline is occasionally noted, but generally the outlines of the mineral are very irregular. The inclusions are microcline, magnetite, and the long, very perfect needles which have been referred to rutile. These occur with a definite orientation in three planes, making angles of 600 with each other. The pronounced cracks which are seen to cross from one crystal to its neighbor, without interruption, are an indication of pressure phenomena in the magma after its consolidation. These cracks are notable as containing microscopic dendritic growths, which are analagous to the arborescent forms of M110 2 , and possibly identical with them. Biotite is present in brown patches. It sometimes shows a metallic luster. The lamellae are often bent or broken. The absorption is strong. Rays vibrating at right angles to the cleavage are brownish yellow. The figtire is nearly that of a uniaxial substance. The effects of weathering are apparent in the chloritized lamellae, and in the rims of chlorite which sur - round the mineral. Plagioclase feldspar is very generally present but only in small amounts. It is invariably finely twinned on the Albite law, while lamellae, due to Pericline twinning, may often be observed. Carlsbad twins are extremely rare. In the absence ib .4, le REPORT OF THE VERMONT STATE GEOLOGIST. 55• of any section perpendicular to M (oio) which might show Aihite lamellae in the two halves of a Carlsbad twin, the optical determination as oligoclase, with the composition Ab 3An 1 , rests upon the observed symmetrical extinction angles of 6 ° or less. The plagioclase is much more deeply weathered than the microdine, and its substance is commonly clouded over by earthy alteration products. The twinning lamellae often show the effects of pressure, as being noticeably bent. MicrogranitiC intergrowtlls with little club-shaped pieces of quartz occur, all the quartz crystals extinguishing simultaneously. Muscovite is an imortant constituent of the rock. It may even be more abundant than the biotite, with which it is so generally associatedl. When originally present it occurs in broad flakes. Much of it is, however, secondary, as resulting from the weathering of the feldspar. In such cases it is in small irregular patches and scaly aggregates. The muscovite shows faint absorption. The axial angle is wide. Apatite is found sparingly in short, blunt prisms, which give the usual six-sided basal sections. Magnetite occurs, but not abundantly, in very irregular grains. Pieces .2 mm. across are seldom noted. Titanite exhibits perfect lozenge-shaped sections .25 mm. in diameter. It is one of the more rarely found accessory minerals. Tourmaline occurs at one locality. The prismatic crystals of this mineral may be i cm. in length. Original epidote in small needles is occasionally met with and minute amounts of secondary calcite are found. A chemical analysis of the darker granite collected in the area south of Millstone Hill yielded the following percentages: I. II. Si02 .............................69.89 68.55 Ti0 2 .................................. A123 C) .............................i.o8 16.21 Fe 203 ............................. 1.04 2.26 1.46 FeO MnO...................................
56 REPORT OF THE VERMONT STATE GEOLOGIST. MgO............................. CaO .66 104 .............................. 2.07 2.40 Na20............................... K20 7 4.08 .............................. 4.29 1.-I0 at 1100 4.14 . .31 H 20 ignit ..........................23 P20 ............. ................ Trace 99.13 I. Granite. Barre, Vt. G. I. Finlay anal. 99.76 II. "Shap" Granite. England. J. B. Cohen anal. Q. J. G. S. XLVII. 276. An analysis of the English "Shap" granite, made for H. Harker, is put beside it for reference. By the aid of the micrometer eye-piece three other varieties of the granite, as noted below, were measured for a quantitative estimate of their mineralogical contents. It is believed that the figures thus obtained, after calculating the percentage by weight for each constituent, are roughly expressive of the range in mineralogical composition throughout the granite mass. I II III. Microcijue ..................... 47.9 Orthoclase 56.8 ...... 38.4 .............. Plagioclase 7 - 5 2.1 8.9 .................... I 5 Quartz .3 ........................ 3 .7 26.0 28.5 Biotjte 18.4 .............................10.2 i\Iuscovjte 26.1 ..................... 3.3 . I Titanite .2 ........................... .6 Magnetite 2.6 ............................ . .0 Apatite ....................................5 99.8 99.6 99.8 I. Acid Granite Southern end of Millstone Hill. II. Granite. Medium Stock. Eastern slope of Millstone Hill. III. Basic Segregation. Granite. Northern slope of Mill- :stone Hill. REPORT OF THE VERMONT STATE GEOLOGIST. 57 The most striking variations from the normal quality of the granite throughout its mass are found in rare miarolitic pockets. One of these, which is notable as showing three distinct types of rock side by side over a very small space, is figured in Plate VIII. In the center of the picture, in the upper half of that portion of the granite defined by its contact with the schist on the left, and the vertical crack in dark shadow, on the right, a rudely V-shaped outline may be seen, delimiting a space of about two square feet. At the top smoky quartz is found. Its presence is significant, since the observation has been made by Dr. A. A. Julien that the occurrence of smoky quartz in the central portions of many pegmatitic veins marks the last stages of vein filling in such cases. At the bottom, in the point of the V, the excessively coarse-grained variety of granitic rock, which makes up the miarolitic seam above referred to in Messrs. Lamson and \Vells quarry, is developed. The sides of the V carry a very fine-grained facies of the granite, lacking in the usual biotite, but abundantly supplied with muscovite. Between the patches of this muscovite-bearing rock on either side, and sharply outlined against them, as showing no gradual changing over to their substance, or into the coarsegrained member in the lowest portion of the V, a porphyritic facies of the granitic magma occurs. In the hand specimen this rock is bluish by reason of the color of its ground-mass. Phenocrysts of white feldspar are far more abundant than those of quartz. Biotite in minute plates, strewn everywhere through the rock, is seen to he the most important dark constituent. Under the microscope large porphyritic crystals of microdine and quartz, with much less orthoclase and plagioclase, and abundant biotite, appear in a coarse holocrystalline groundmass, composed of quartz and feldspar, in irregular grains. The quartz phenocrysts give at best only a rough approximation, on basal sections, to hexagonal outlines. Microcline usually shows but slight evidences of twinning. Its substance is glassy, rarely kaolinized. Some orthoclase is present in rudely auto-
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