416 Scientific Proceedings, Royal Dublin Society. 



alteration resulting, in many cases, in the production of an earthy 

 state ; and chemical examination proves that in such cases the rocks 

 are hydrated. 1 Highly instructive, as bearing upon the formation 

 of minerals of this class, are the researches of Daubree 2 upon the 

 production of zeolites, during the historic period, by the action of 

 thermal waters, at a temperature not above 70° C, upon the ma- 

 sonry of the ancient Roman baths at Plombieres. 



Water containing C0 2 is not considered by F. Senft 3 to play a 

 part in the production of zeolitic minerals out of Labrador- felspar, 

 &c. The evidence, he holds, suggests rather the action of pure 

 water, which, saturating atom by atom the substance of the pre- 

 existing minerals, causes their complete hydration. The zeolitio 

 mass thus formed may either remain undisturbed, or it may gra- 

 dually become dissolved, and thus travel until evaporation or 

 other causes invokes crystallization. A remarkable fact in the 

 history of zeolites, has lately been made known by the researches 

 of Messrs. Murray and Renard. 4 They found that the decompo- 

 sition of volcanic detrital material goes on at low temperatures in 

 the depths of the ocean, transforming a basic silicate into a crys- 

 talline zeolite on the one hand, and the characteristic red clay of 

 deep sea-deposits on the other. The temperature at which this 

 alteration proceeds is approximately 0° C. The investigations of 

 Friedel and Sarrasin are also remarkable as bearing not only upon 

 the production of felspars in the wet way, but also upon the pro- 

 duction at will, under similar conditions, of a felspar or a zeolite. 5 



The properties of minerals of this class were first studied by 

 Cronstedt, who applied the general term "zeolite" on account of 

 the fretting and swelling they exhibited before the blowpipe. 6 

 But it was Bergmann who first observed another characteristic 



1 A. Daubree, " On Points of Similarity between Zeolitic and Siliceous Incrustations 

 of Becent Formation by Thermal Springs, and those observed in Amygdaloids and 

 other altered Volcanic Eocks." Quart. J. Geol. Soc. Lon., vol. 34, 187S, p. 80. 



2 A. Daubree, Etudes synthetiques de Geologie experimentale. Paris : 1879, p. 180. 



3 Die krystallinischen Felsgeinengtheile. Berlin : 186S, p. 622. 



4 Les caracteres microscopiques des cendres volcaniques et des poussieres cosmiques 

 et leur role dans les sediments de mer profonde. Bull. Musee Boy. d'hist. nat. de Bel- 

 gique iii., Bruxelles : 1884 ; also "Volcanic Ashes and Cosmic Dust," Mature, xxix., 

 p. 585 (1883-4). 



5 Cf. Michel-Levy et Fouque, Synthese des mineraux et des roches. Paris : 1882. 



6 Kong, Vet. Acad. Hand!., 1756, p. 120 ; and an Essay towards a System of 



