Hyland — On the Mesolite (Gaiactite) of Kenbane Head. 415 



of phillipsite — an explanation offered by Stock 1 for the occurrence 

 of 4-46% of this element in a zeolitic mineral, which is found in 

 the cavities of the nepheline-dolerite of Lobauer Berg in Saxony. 2 



The analyses of gaiactite furnished by Heddle show that the 

 amount of CaO may become very trifling : a transition to true 

 natrolite is thus effected. According to the same writer, it is the 

 small portion of lime which gives to these zeolites "their white- 

 ness and opacity, and doubtless prevents their assuming the definite 

 crystalline form which the pure mineral under favourable circum- 

 stances adopts." 3 



All zeolites are decomposition-products, and seem to owe their 

 origin to the alteration of minerals such as plagioclase, nepheline, 

 &c. 4 Chemically, these minerals may be said to represent the 

 silicates of the nepheline and felspar group, combined with water. 

 In fact, many zeolites seem to be hydrated plagioclases ; 5 a compa- 

 rison of the formulae will render the probability of this apparent. 



From the circumstance of finding zeolites disseminated through 

 the minutest cavities of the mass of volcanic rocks, it was formerly 

 believed that these minerals were formed by segregation in the 

 mass of the rock, and that they were enabled to retain their water 

 at a high temperature, under the influence of pressure. The 

 results of subsequent observations, however, have led to the belief 

 that the deposition of zeolites is a direct consequence of the infil- 

 tration of water through the mass of the rock. This water, react- 

 ing upon the constituents, will give rise to various species of 

 minerals. The first effect will be that of decomposition, and 

 subsequently of reconstruction, with the formation of new com- 

 pounds. Zeolitic minerals may therefore be considered a kind of 

 "extract" of the rocks so subjected to continued lixiviation — a 

 view that is borne out by the more or less altered condition usually 

 observed in the rocks most abounding in this class of mineral — the 



1 J. Stock, Die Basaltgesteine des Lobauer Berges. Tschermak's Mitt. : 1888, 

 Bd. ix., Heft. 6, p. 453. 



2 Schneider, Geogn. Beschreibung d. Lobauer Berges. Abh. der naturf. Ges. zu 

 Gorlitz. : 1868, Bd. xiii., p. 42. 



3 Heddle, 1. c, p. 274. 



* Whitman Cross and "W. F. Hildebrand, "Contributions to the Mineralogy of the. 

 Rocky Mountains." Bull, of the U. S. Geol. Survey, 1885, No. 20. 



5 Tschermak, Lehrbuch der Mineralogie. Wien: 1889, 3te. Aufl., p. 495. 



