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FIFTEEN samples of erratics from the shingle deposit
at Stonar, Kent, submitted by Mr. W. P. D. Stebbing, have now been
examined, and Dr. J. V. Harrison, to whom I am most grateful, has helped
greatly in the following description. The numbers of the samples refer
to those given to them by their finder.
1. PINK HORNBLENDE-GRANITE. Medium-grained, with felspars
up to about 5 mm. in size. The rock consists mostly of quartz and
orthoclase, though a little oligoclase is also present. The quartz and
orthoclase show micropegmatitic intergrowth. Both hornblende and biotite
are present, associated with magnetite and chlorite. Epidote also
occurs, and zircon and apatite are accessory.
2. MICACEOUS GRIT. Slightly schistose and veined with
quartz. The quartz appears to be clastic, and is coated with limonite
and haematite. Magnetite is also present. Muscovite and bleached biotite
are common, and may be of secondary origin.
3. LEUCO-GRANITE, with chlorite veins. Quartz is common,
and the felspars are orthoclase, perthite and oligoclase, these three
being present in roughly equal proportions. No primary ferromagnesian
minerals seen, but needles of magnetite may be secondary after biotite. |
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Apatite, zircon and limonite are accessory, and green
veins appear to consist of chlorite.
4. LEUCO-GRANITE, but not the same rock as No. 3, as this
specimen contains different felspars from the other. It is a white rock,
consisting of large allotriomorphic crystals of felspar, up to 3 or 4
mm., set in a mosaic of smaller crystals of quartz and felspar, about
0·5 mm.across. About half the rock is made of quartz, and the remainder
is mostly microcline and perthite, with some albite. No ferromagnesian
mineral seen in the slide, and apatite and haematite seem to be the only
accessories.
5. FINE-GRAINED GREY SANDSTONE. Under the microscope this
rock is very even-grained, the average size being about 0·1 to 0·2 mm.
Almost entirely made of quartz, though a few flakes of muscovite also
occur. Grains of zircon, and occasionally of rutile and tourmaline are
also scattered through the rock. These are probably of detrital origin.
6. DEVITRIFIED PORPHYRITIC PITCHSTONE. In hand specimen,
this is a purple rock showing porphyritic crystals up to about 5 mm.
Under the microscope, andesine proves to be porphyritic, but the
microcrystalline groundmass is probably rich in orthoclase, as its |