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V.—Note on some Pebbles in the Basal Conglomerate of the Cambrian at St. Davids

Published online by Cambridge University Press:  01 May 2009

T. G. Bonney
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Extract

More than one kind of rock, as we learn from Dr. H. Hicks and other writers, occurs in the conglomerate which forms a well-marked base to the Cambrian system at St. Davids. Sometimes the pebbles are mainly vein quartz, sometimes felstone predominates, but occasionally, as in the neighbourhood of Nun's Chapel Bay, quartzites (using the term rather generally) are not uncommon. At one place, not far from a quartz-felsite dyke, these are rather large, occasionally about a foot in diameter. From this locality, while spending a few days at St. Davids in 1882, I brought away specimens of three of the most marked varieties of quartzite, of which I had slices prepared, thinking that as examples of rocks which were probably far from modern at the beginning of the Cambrian age, their structures might be instructive.

Type
Original Articles
Information
Geological Magazine , Volume 6 , Issue 7 , July 1889 , pp. 315 - 318
Copyright
Copyright © Cambridge University Press 1889

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References

page 315 note 1 Q.J.G.S. vol. xl. p. 567, id. (Blake) p. 294.Google Scholar

page 317 note 1 e.g. N.W. Scotland, Hartshill, Lickey, Wrekin, Stiper Stones, N. Wales, Cherbourg, the Ardennes, pebbles from Carboniferous and Triassic rocks, etc.

page 317 note 2 Described in Q.J.G.S. vol. xliv. p. 32.Google Scholar

page 317 note 3 Q.J.G.S. vol. xlv. pp. 87–105.Google Scholar

page 317 note 4 See Q.J.G.S. vol. xxxix. p. 47Google Scholar (note). Prof. Blake (id. vol. xliv. 475, 508) calls this a quartz knob, and seeks to show that it has more resemblance to a vein product than to a rock of clastic origin. I cannot, of course, answer for the specimen which he examined, but can only say that if my slide does not represent a quartzite, I have never seen one.

page 318 note 1 See, for instance, Q.J.G.S. vol. xliv. p. 16.Google Scholar

page 318 note 2 United States Geological Survey, Fifth Annual Report, 1883–4.

page 318 note 3 I must not be supposed to assert that a hard, fast, and universal line can be drawn between Archæan and Palæozoic (or Cambrian and Pre-Cambrian) any more than between Palæozoic and Mesozoic or between any other geological groups, systems, or classificatory divisions.