[429.0] 
XVIII.—ON THE OCCURRENCE OF CRYSTALS OF SALT 
(CHLORIDE OF SODIUM) IN CHERT FROM THE 
CARBONIFEROUS LIMESTONE, sy EDWARD HULL, 
M.A., F.R.S., PROFESSOR OF GEOLOGY IN THE RoyAL COLLEGE OF 
SCIENCE FOR IRELAND; DIRECTOR OF THE GEOLOGICAL SURVEY OF 
JTRELAND. 
[Read December 16, 1878. ] 
THE occurrence of salt crystals in the fluid cavities of granitic 
and plutonic rocks has been brought to light by several petro- 
logists. Professor Zirkel has noticed them in the granite of 
Goatfell in Arran in Scotland ;* Mr. Sorby in the quartz of the 
granite of Trevalgan near St. Ives,f and the Abbé Reynard in 
the quartziferous diorite of Quenast.} Other examples might be 
cited. These objects are amongst the most curious and beautiful 
which the study of the rocks under the microscope has revealed. 
During my investigations of the structure of the chert-beds of 
the Carboniferous Limestone of Ireland I had on several oc- 
casions noticed cubical forms, often black, but sometimes slightly 
translucent ; these I generally supposed to be crystals of pyrites or 
the cavities left by them, the darkness of the sides being due 
partly toa coating of probably carbonaceous dust, and partly to 
the absorption of the light when the walls of the crystal are 
placed in certain directions with reference to the incident rays. 
A short time since, my friend Professor A. Reynard, of Louvain 
when kindly sending me a thin section of the “ Diorite quartzi- 
fere” of Quenast, which he and M. Chevalier de la Vallée Poussin 
have made the subject of special examination, accompanied this 
specimen by another of black chert from Ballymote, Co, Sligo, 
which, on examination under the microscope, I observed to con- 
tain numerous small, but beautifully perfect cubes, generally with 
darkened sides, but with translucent centres. I could not doubt 
the nature of these little cubes, after comparing them with those 
contained in the fluid cavities of the quartziferous diorite of 
Quenast, and with figures and descriptions given by Dr. Ferd. 
Zirkel in his valuable handbook§. My colleague, Professor 
O'Reilly, M.R.I.A., has been also kind enough to examine the 
specimen, and admits the high probability that the cubical 
* Zirkel. Die mikroscopische Beschaffenheit der Mineralien, p. 55, Teipsig (1878). 
t Ibid, p. 57. 
¢ “Memoire sur les caractéres minéralogiques des roches de la Belgique, &c.,” par 
M. M. Ch. dela Vallée Poussin et A. Reynard, S.I., Bruxelles (1876). 
§ Die Mineralien und Gesteine, p. 55. 
