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as in the inclosed core (figure 9, in which the upper and lower halves are cut at different angles). Another striking structure is the micropegmatite, the grains showing it being often very abundant (figure 10).

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Distribution and Field Relations-As already stated, a very large proportion of the area occupied by the crystallines of the Mine la Motte district is granite. The extent of the principal mass is equivalent to an area of nearly 50 square miles. It forms a part of the great central granite mass of the region. From Knob Lick it reaches westward beyond the limits of the sheet. Its north and south extension is from Doe Run to the Silver mines. On all sides, small, isolated areas also occur. On the north the central granite area is bounded by the St. Francois river, and on the east by paleozoic sandstones. Along the river bluff, in the vicinity of Syenite, the rock is red in color, but a short distance away it assumes a bluish tint. In some places, as south of the Milne and Gordon quarry, it appears to pass into a porphyry phase, but farther south the blue variety is found. Still further south ward, at Knob Lick hill, a fine-grained rock occurs, which gradually changes into a porphyry before the summit is reached, so that the highest portion of the elevation is composed of the ordinary, red porphyry.

The genetic relations of the red and blue granites are of much interest, but the difficulties arising in attempting to trace their boundaries are great, on account of the heavy covering

of soil which exists over much of the surface between the river bluff and the steeper parts of the hill, which is about half a mile away. When the granites were first carefully observed, it seemed quite possible that they are not the same mass, but belonged to two different periods of eruption, but so far as can be observed the evidence is quite conclusive that the red granite, the blue granite and the porphyry are one in origin. In considering more carefully the distinguishing characters of the two periods, it is manifest that there is only a slight difference in the color of the feldspar. Each variety is composed essentially of quartz, feldspar and a small amount of biotite, in about the same relative proportions; and has practically the same texture.

South and west of Knob Lick hill the granite varies considerably, sometimes closely resembling porphyrys, but at others is very coarsely crystalline. Wherever a ridge or hill occurs the texture invariably becomes finer, and the rock correspondingly more like porphyry. The only important change in composi tion is a variation in the amount of biotite. For a few miles south of the locality just mentioned this mineral seems to be wanting. But farther to the west, especially in the vicinity of the Silver mines, and on Stout creek beyond the limits of the sheet, it is almost as abundant as around Syenite. To the northwest of Syenite the biotite is comparatively abundant for a few miles, then it gradually diminishes in quantity, and disappears altogether in the northwest part of the area.

North of the river, opposite Syenite, sandstone prevails as the surface rock, but here and there erosion has been removed is so that considerable granite crops out. Such is the exposure at the old Allen and Veith quarry on Doe Run (Tp. 34 N., R. V. E., Sec. 36 ). A red granite occurs quite similar to that found at Syenite; and also at the St. Louis quarry a little farther to the northeast. At the latter point the exposure is very small, the sandstone coming to the walls of the quarry. Among the small isolated areas is one on Rock creek, north of Mine la Motte, which has a very striking black and white mottled appearance, the peculiarities in the color of which is

due to an almost pure white feldspar and an intensely black biotite.

On the Mine la Motte estate, just east of the dressing works, is a small granite hill of a few acres in extent, and still farther to the east is a larger mound of the same kind of rock. These are the most easterly exposures within the limits of the sheet. Both are comparatively rich in biotite, and the feldspar is red, giving it a very pronounced red and black color. texture it is finer than the typical granite.

East of Mine la Motte station are three low granite hills. The Little St. Francois river passes between the northern two, and the Belmont branch of the St. Louis, Iron Mountain & Southern railway between the southern two. The granite in the two Scale 504 Hon

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FIGURE 11. Dispositon of Sedimentaries on Crystallines.

northern areas is about the same and carries a considerable amount of biotite. The rock in the southern one of the three hills has little if any biotite and is not so coarse-grained. This is the place, below the railroad bridge, where occurs the exceptionally good exposure, showing the juncture between the granite and the overlying stratified rocks. The river makes a small horse shoe curve and cuts off the southeastern portion of the granite. On each side of the curve, at points not over two hundred yards apart, the relations are well displayed (plate vi). The crystalline rock is a part of an old pre-Cambrian bill. Its surface is paved with rounded boulders of granite, from a few inches up to several feet in size. These are embedded in a compact sandstone which overlies the granite to a thickness of a dozen feet or more, becoming somewhat thinner, however, uphill. The relations are shown above (figure 11).

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