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II.—On Analysis of White Chalk from the County of Tyrone, with Note on the Occurrence of Zinc therein, and in the overlying Basalt1

Published online by Cambridge University Press:  01 May 2009

Edward T. Hardman
Edward T. Hardman
Affiliation:
Geological Survey of Ireland; Associate of the Royal College of Science, Dublin.
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Extract

I was led to make this analysis with the view of determining if possible whether the extreme hardness of the Irish Chalk were due to either a chemical, a mechanical, or a calorific alteration, from the influence of the overlying basalt. If it were owing to chemical change, we should expect to find a large per-centage of silicates and a diminution in the amount of lime; if to the influence of heat, carbonic acid would be driven off, silica would be in excess, and the presence of the insoluble bases, such as the peroxide of iron, oxide of manganese, and alumina, would become more apparent; while if a mechanical cause or pressure were the reason, no change would take place in the relative amount of the constituents. I believe the result seems to show that the induration of the Chalk must be set down chiefly to the last agency,—if to anything apart from the original formation of the rock,—and that the power of alteration exerted over it by the heat of the molten basalt has been small indeed. At the same time a certain addition has been made to it by means of water holding chemical bodies in solution.

Type
Original Articles
Information
Geological Magazine , Volume 10 , Issue 112 , October 1873 , pp. 434 - 438
Copyright
Copyright © Cambridge University Press 1873

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Footnotes

1

Read before the Royal Geological Society of Ireland, June 11, 1873.

References

page 435 note 1 Fletcher's hot-blast gas blow-pipe was used, and by its means a comparatively large quantity of the powdered chalk and of the basalt (see post) could be treated.

page 436 note 1 The spot from which the basalt was obtained is about 300 yards from the Chalk Quarry.

page 436 note 2 Mr. Thomas Davies, F.G.S., of the Department of Mineralogy in the British Museum, informs me that Professor Scacchi, of Naples, states that Blende (Zn S) occurs occasionally—in association with galena—in the volcanic breccias of Monte Somma.—Edit. Geol. Mag.

page 436 note 3 Dana's System of Mineralogy (1868), p. 153.

page 436 note 4 Ibid. p. 135.

page 436 note 5 Ibid. p. 50. (Here, too, the removal of the obnoxious zinc from the lead is a most difficult problem. See Percy's Metallurgy of Lead, p. 325.)

page 436 note 6 Ibid. p. 49.

page 437 note 1 Prof. Hull “On the Structure of Trap Rocks,” Geol. Mag., April, 1873.

page 437 note 2 Manual of Geology (Jukes and Geikie), p. 63. (Article by W. K. Sullivan, Ph.D.).

page 437 note 3 Cotta, Rocks Classified and Described, p. 61.

page 437 note 4 System of Mineralogy (1868), p. 17.

page 437 note 5 The above bracketed portion was added after the paper had been read, as the possibility of zinc occurring in the igneous rocks was disputed at the time. The method of analysis was also said to be unreliable; but it is a far more certain test for small quantities, than the wet process. By it a metal was obtained. This was white and brittle; it dissolved readily in dilute hydrochloric acid; and the solution of the metal heated on eharcoal with nitrate of cobalt gave a very distinct bright green incrustation. There is but one metal that exhibits all the above characteristics, and that is zinc.

Voltzite (Zn S + ZnO) occurs at Rosieres, near Pont Gibaud, Puy de Dome (Dana's Mineralogy, p. 50). No mention is made of the rock in which it is found; but in Scrope's “Volcanoes of Central France,” both in the maps and in the letter-press (pp. 56, 57), the rocks of that neighbourhood are shown to be granite and recent basalt; no other than igneous rock is laid down on the maps, nearer than about 18,000 metres. (Edition 1858.)

page 437 note 6 Bischof mentions that small pieees of clay-slate caught up in lava flows were afterwards found to be quite unaltered.

page 437 note 7 Manual of Geology (Jukes and Geikie), p. 69.

page 438 note 1 The Chalk of Tyrone is in fact curiously shattered and split up into small irregular parallelopipeds, which appears to be due to more than ordinary jointing. The great pressure may have had something to do with it.

page 438 note 2 Journ. Royal Dub. Soc., July, 1860.

page 438 note 3 A quantity of the Antrim Chalk is, however, exported to England for manufacture there.