E. T. Hardman — White Chalk of Tyrone. 437 



so abundant in that rock. This is perhaps the most likely to happen ; 

 but in either way, most of the zinc minerals might have been 

 brought in. There is nothing therefore remarkable in the fact of 

 the metal being detected ; and it might be met with oftener were it 

 looked for during the analysis of rocks, just as other metals known 

 to exist in certain rocks seldom appear in their published analysis. 



[It can only be a matter of surmise as yet with regard to the form 

 in which the zinc exists in the basalt. A possibility might be sug- 

 gested, however, that some of the magnetite which is found as an 

 accessory mineral in that of Antrim' may be of the variety Frank - 

 linite. Some of the spinels also, to which this is allied,^ and which 

 themselves number a zinc compound, have been found in vesicular 

 cavities of the volcanic rocks of Monte Somma;^ and at least one 

 instance is given of the discovery of metallic zinc in basalt. It is 

 said, by G. Ulrich, to have been found by a quarryman in a geode in 

 basalt, near Melbourne, Victoria, associated with Smithsonite and 

 cobalt bloom ; the specimen weighed 4^ ounces. Dana, however, 

 considers the account to be somewhat doubtful.*}^ 



With this exception, the analysis shows little that could be ascribed 

 to chemical alteration, nor has there been any loss of carbonic acid, 

 so that the original heat of the superincumbent basalt, which has 

 been so often relied on as the means of hardening the chalk, and 

 reddening the flints, must be quite out of the question. Perhaps a 

 great deal too much power has been ascribed to this agent ; as may 

 be conceded when we recollect the imaltered condition of the inter- 

 vening leaf-beds and lignite, which do not appear to have been 

 affected by it, although naturally very susceptible of its influence.^ 



But when it is remembered that in the Hebrides the basalt reaches 

 a thickness of between 3000 and 4000 feet,'' and that the Irish basalt, 

 although now but fi'om 500 to 1200 feet thick, may have had 

 similar pi'oportions, there is no difficulty in referring the consolida- 

 tion of the Irish chalk to pressure alone; for, taking the original 



1 Prof. Hull " On the Structure of Trap Rocks," Geol. Mag., April, 1873. 



2 Manual of Geology (Jukes and Geikie), p. 63 (Article ty W. K. Sullivan, Ph.D.). 

 " Cotta, Rocks Classified and Described, p. 61. 



* System of Mineralogy (1868), p. 17. 



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 charcoal 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 -j- 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.) 



^ Bischof mentions that small pieces of clay-slate caught up in lava flows were 

 afterwards found to be quite unaltered. 



' Manual of Geology (Jukes and Geikie), p. 69, 



