E. T. Uardman — Substitution of Zinc for Magnesium. 201 



same enlarged, c. Portion of a branch still further enlarged. From the 

 Hamilton Group. 

 Fig. 21. — JRetepora FMlUpsi, 'Nich. a. Fragment of the natural size. b. Fragment 

 in which the outer non-celluliferous layer has been removed, showing the bases 

 of the cells, enlarged. From the Corniferous Limestone. 

 Fig. 22. — Fenestella magnifioa, Nich. a. Portion of the polyzoary, natural size. 



6. Fragment of the same, enlarged. From the Corniferous Limestone. 

 Fig. 23. — Fenestella margmalis, Nich. a, A fragment, of the natural size, b, A 

 portion of the same, enlarged, c. A portion of a branch still further en- 

 larged, viewed from behind, and showing the large marginal and the minute 

 central cells. From the Corniferous Limestone. 

 Fig. 24. — Fenestella fiUformis, Nich. a. Fragment, natural size. b. Portion of the 



same enlarged. From the Corniferous Limestone. 

 Fig. 25. — a. Spirorbis omphalodes, Goldfuss, of the natural size and enlarged, b. 

 Spirorbis Arkonensis, Nich., of the natural size and enlarged, e. A dextral 

 example of the same, enlarged. From the Hamilton Formation. 

 Fig. 26. — Ortonia intermedia, Nich. a. One of the tubes enlarged, h. Another 

 example in which the annulations are greatly extended laterally, enlarged. 

 From the Hamilton Formation. 



III. — On the Substitution of Zinc for Magnesium in Minerals. 



By Edward T. Hahdman, F.R.G.S.I.i 



Of the Geol. Survey of Ireland, Associate of the Royal College of Science, Dublin. 



IN the Geological Magazine of last October ^ I have noticed the 

 occurrence of zinc in small quantities in the White Limestone 

 (Chalk) of the Co. Tyrone, and also in the overlying basalt. "When 

 that paper was read,^ exception was taken to the latter part of it, on 

 the ground that the metal in question had never been known to exist 

 in rocks of igneous origin ; but as my analysis seemed perfectly 

 trustworthy, I saw no reason to modify it on the objections 

 then put forward. Since then, whilst making an investigation — 

 with a different object — on a specimen of basalt from another 

 locality in the neighbourhood of that whence the one originally ex- 

 amined was obtained, I succeeded in again finding zinc in it ; and I 

 think it well to mention the fact now, as an addendum to, and as con- 

 firming my former statement. 



The specimen was procured at a place called Curglasson, near 

 Stewartstown, from the heart of a quarry by the roadside ; being- 

 more than a mile north of the spot whence the basalt referred to in 

 my former communication was obtained. The rock was carefully 

 examined qualitatively for all the metals precipitable from an acid 

 solution by sulphuretted hydrogen. Iron and alumina were then 

 precipitated, and the filtrate examined with the utmost caution. On 

 the addition of ammonium sulphide, a distinct white precipitate 

 appeared. As it was probable that a little lime might have been 

 brought down by a trace of carbonic acid in the re-agents, the pre- 

 cipitate was filtered off, re-dissolved in hydrochloric acid, and re- 

 precipitated, (a) It was again re-dissolved, evaporated to dryness, 

 and strongly ignited, to drive off ammonia, etc. A small quantity of 

 the residue, which was deliquescent, was mixed with carbonate of soda, 



^ This paper formed part of a communication read before the Eoyal Irish Academy, 

 January 26, 1874. 2 Vol. X. page 434, 



^ Before the Eoyal Geological Society of Ireland, June 11, 1873. 



