Report on Erratics from Stonar, Kent

By D. Baden-Powell, B.Sc.

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. 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 [pg51]refractive index is less than that of Canada Balsam. It is frequently spherulitic. No quartz was seen in the slide. Biotite is both porphyritic and scattered through the groundmass, and sometimes shows alteration to chlorite, and to magnetite. Accessory zircon occurs.

7. Pale Brown Grit, the hand specimen showing distinct yellowish coating of some of the grains. The slice shows that almost all the grains are of quartz, and two distinct grain-sizes occur, a few of these being up to about 5 mm., but the majority measure only 0·2 to 0·5 mm. Sericitic mica has grown between the grains, and there are rare small detrital crystals of zircon.

8. Veined Chloritic Schist. A fine-grained bluish-green rock in the hand specimen. Seen under the microscope to consist of rather angular quartz grains, averaging 0·1 to 0·2 mm. in size, and showing strain-extinction. These grains are packed round by abundant chlorite, with a few muscovite flakes. A conspicuous vein consists of quartz and calcite, and accessory zircon is present both in the schist and in the vein. No felspars were seen in this rock.

9. Aplite. Fine-grained speckled rock which consists of allotriomorphic crystals of quartz, orthoclase and oligoclase. These three minerals are present in approximately equal proportions, and are also equal in grain size, about 0·3 mm. The rock also contains a little muscovite and zircon, and is stained by limonite.

10. Basic Diorite, or possibly Camptonite. Dark medium-grained, rather weathered rock, made up almost entirely of zoned felspars and hornblende in approximately equal quantities. The saussuritization of the basic centres of the felspars has gone too far for them to be identified, but the fresh rims suggest oligoclase. The hornblende shows green, brown and yellow pleochroism. Ilmenite and apatite also occur.

11. Muscovite-Biotite Granite, fine-grained. The slice shows that quartz is abundant, and the chief felspar is orthoclase, with less common oligoclase and rare microcline. Both muscovite and biotite are common, and zircon and apatite are accessory. The grains are allotriomorphic, the quartz showing sutured margins, and the size of grain is about 1 mm. maximum.

12. Quartzite, with white spots and partly stained red. Made up of coarse quartz grains, mostly from 0·5 to 3 mm. in size, some coated with a mineral resembling muscovite. This coating may account for the white spots visible in the hand specimen. Limonite and haematite are both present. The rock appears to have been slightly sheared and brecciated.

13. ? Biotite-Aplite. A very weathered pink rock consisting mostly of quartz and kaolinized felspar, some of which may have been oligoclase. Very altered biotite is subordinate to the other minerals. [pg52]

14. Epidotised Porphyritic Monzonite. In the hand specimen this is a black and yellow rock in which porphyritic crystals of felspar up to nearly 10 mm. length are easily seen. The slice shows that the rock consists of euhedral zoned plagioclase crystals with decomposed cores, perhaps originally labradorite, with oligoclase margins, set in a matrix of fresh orthoclase and subordinate quartz. Hornblende is common, and is associated with chlorite after biotite and a little ilmenite. Epidote occurs throughout the rock. Apatite is accessory.

15. Quartz Diorite. Medium-grained rock, with grain size up to about 1·5 mm. Quartz is common, but rather local, in the slide, and the felspar is mainly andesine with some orthoclase. Hornblende is abundant, but is much associated with chlorite and iron ores, and may be decomposed augite. Accessory minerals are limonite and apatite.

Most of the rocks in this collection resemble types known in the Drift of East Anglia, but the latter have not yet been worked out in sufficient detail to say definitely whether these Stonar erratics might have come from there. It should be pointed out that in any case they may also have been derived from the Thames Plateau and River Gravels, but it would not be easy to distinguish between the Thames erratics and those of East Anglia, because in some cases these two groups have been derived from a common source.