Journal - South African Institute of Mining and Metallurgy . ount for these observeddifferences, fair samples of each type of pebblewere taken and analysed with the followingresults :— Local ImportedPebblea Pebbles. % % Loss on ignition (, CO.,, etc.) ... ..-. 0-46 100 Silica (Si02) ... ... 97-20 9678 Iron oxide and alumina (Fe.,C3 andAl2Og)... ... 1-95 1-86 Lime(CaO) ... ... 0-22 026 Magnesia (MgO) ... ... 0T6 0-07 The composition of the two classes, therefore,was practically identical, and furnished no clue tothe different properties or behaviour. As alreadystated, the local pebbles were
Journal - South African Institute of Mining and Metallurgy . ount for these observeddifferences, fair samples of each type of pebblewere taken and analysed with the followingresults :— Local ImportedPebblea Pebbles. % % Loss on ignition (, CO.,, etc.) ... ..-. 0-46 100 Silica (Si02) ... ... 97-20 9678 Iron oxide and alumina (Fe.,C3 andAl2Og)... ... 1-95 1-86 Lime(CaO) ... ... 0-22 026 Magnesia (MgO) ... ... 0T6 0-07 The composition of the two classes, therefore,was practically identical, and furnished no clue tothe different properties or behaviour. As alreadystated, the local pebbles were evidently of com-paratively coarse structure, so that the next step June 1908 A Laboratory Crnnpa Tube Mill Pebbles. to have thin sections prepared for microscopicexamination in order that the internal structure could be studied, and this, as shown in thephotographs, furnishes, I think, the explanationsought for. Both photographs were taken with polarisedlight and show a magnification of 75 dlamiIn No. I., a local pebble, it is seen that the. FIG. L—LOCAL PEBBLE section is not only composed of comparativelycoarse grains with nearly smooth edges (as dis-tinct from photo III.), but that the grains arenot of uniform size, in fact, the mass is composedof mosaic secondary quartz with a few specks ofother minerals, and is really a quartzite. Thegrains do not appear to be cemented together,but are held together, though not strongly,mainly by interlocking, thus creating difficulty inpreparing the slide as they easily separated duringgrinding. This accounted for the comparativebrittleness of these pebbles, as the difference insizes of grains may account for uneven Xo. II., on the other hand, an importedpebble, the structure is crypto-crystalline andhomogeneous, and in places distinctly is probably composed therefore of the Chalce-donic variety of silica, , a true flint, and nodifficulty was met with in making the greater brittlenes
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