Mineral Industries and Geology of Certain Areas - Vermont Agency ...

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52 REPORT OF THE VERMONT STATE GEOLOGIST. ries" only can be operated, and the output is of small-sized blocks. The more favorable quarry locations are cut by few joints, and pieces of any desired size can he taken out to the limit of transportation. For many feet below the surface the approximately horizontal joint-planes, roughly parallel with the outlines of the granite mass, give rise to a perfect development of the well-known " onion structure " in the quarries. The same tendency may be noted on the ledges in the woods along the western slopes of the hill, where at times four or five Successive steps, cut out by sloping joint-planes in the same ledge, may be seen over a little space. The remarkably perfect and even slickensided planes standing at high angles, which are characteristic of granites everywhere, are often seen in these quarries. They are evidently due to pressure, and fall into two systems, nearly at right angles to each other. Surfaces showing well developed feather-fractures where there is no evidence against their being on the natural joint-planes are occasionally found in the quarries, but there is always a good chance of their having been produced artificially. The effects of weathering are not found deep below the surface in the main portions of the granite mass, but thoroughly weathered shear zones of fractured and useless rock are often met. No well marked schistosity, or gneissoid phase of the granite was observed, but the rock is characterized by the distinct grain which is known to the quarrymen as the "rift." This, with the "heave " and "lift," are the three planes of more easy parting at right angles to each other, which define the blocks as they are taken out. Hand specimens of the granite collected from various points on and about Millstone Hill are widely different in general appearance. They vary little however in the proportions of the constituent minerals. Feldspar, quartz, and mica make up the rock, which is prevailingly medium grained. Its color is gray, or bluish gray. Upon the abundance or scarcity of black mica depend primarily the grades of dark, medium, or light stock known to the trade. Much of the granite on the western and southern slopes of Millstone Hill is relatively so poor in mica r P, I
REPORT OF THE VERMONT STATE GEOLOGIST. 53 as to be quite different in character from the main mass. It is not considered valuable, and but little of it has been quarried. The dark stock, which brings the highest prices, is found over the east-central portions of the hill, and well to the south of it. It tends to become slightly more basic in depth, as well as 7 locally, near the contacts, where large inclusions of the schist • are met with. The darker granite is often finer grained than the light and medium grades. It differs from them in the distinct bluish cast which characterizes its feldspars, a well as in its larger content of mica. In the light colored granite the * feldspar crystals are seen to be hardly tinged with blue. Basic -segregations, known to the quarrymen as "knots," are occasionally found. In them the mica is extraordinarily abundant, and quartz is rare, as compared with the feldspar. The "knots" are seldom above six inches in diameter. They are sporadic in their distribution, so that no portions of the rock can be quar - ried which have any value from their occurrence in it. Miarolitic veins and pockets occur at many points. The quarry operated by Messrs. Lamson and Wells, some distance to the south of Millstone Hill, contains a very coarse grained miarolitic sean-i which runs across its western face. A small pocket of similar character exposed in Messrs. Jones Brothers' quarry is of especial interest, as showing the same coarse grained variety of the rock, in association with the rare porphyritic facies of the granite magma where a ground-mass is developed. A brief description of these variations from the normal quality of the stone will be given later. Examination with a hand lens will very seldom make plain twinning striae on the cleavage faces of the feldspars. Carlsbad twins are seen to he very rare. The quartz is dull in appearance. Biotite is found in small black or rusty brown patches, and silvery scales of muscovite may often be noted. Pvrite is almost never seen. Microscopical study reveals abundant microcline, quartz and biotite, with lesser amounts of plagioclase and muscovite, as the essential constituents of the rock. Some orthoclase is also
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