SPRINGS AND THOSE OBSERVED IN AMYGDALOIDS ETC. 77 



the minute microscopic pores. Such a complete reproduction of 

 phenomena speaks for a common method of origin, which is con- 

 firmed by an examination of the method of occurrence considered 

 as a whole. 



Mode of Formation of Contemporary Zeolite. 



From the circumstance of finding zeolites disseminated through the 

 minutest cavities of the mass of old volcanic rocks, it was at one 

 time supposed that these minerals were formed by segregation in the 

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

 high temperature, under the influence of pressure. The results of 

 subsequent observations, however, have led to the belief that the de- 

 position of zeolites is a direct consequence of infiltration through 

 the mass of the rock ; but as, under ordinary conditions of 

 laboratory experiment, these hydrated silicates could not be repro- 

 duced in the crystallized form, it was considered that a high degree 

 of temperature and a pressure sufficient to prevent the escape of the 

 water of crystallization, were elements essential to their production. 



The above conjectures are untenable in the face of the facts 

 previously set forth, which furnish, so to speak, an experimental de- 

 monstration of the method under which the phenomena in question 

 have actually been produced. 



The Roman masonry, although now of a very compact texture, 

 was originally freely penetrable by water through its entire mass, on 

 account of the essentially spongy character of the bricks when first 

 laid ; and, the absorbed water being forced to circulate by the pressure 

 of the spring, the mass of masonry was not only surrounded but 

 penetrated by the hot mineral water. A current, of feeble strength 

 no doubt, but still continuous, being kept up, the perpetual re- 

 newal of the water, which contains only traces of saline matters, gave 

 rise to the accumulation of deposits in notable quantity, by the 

 accumulated action of a cause of low intensity, but continued for a 

 considerable time. This is a condition difficult of attainment in 

 laboratory experiments, when undertaken in imitation of natural 

 processes ; but its importance when applied to the explanation of 

 different geological phenomena will be readily appreciated. 



The heated mineral water, in virtue of its alkaline constituents, 

 exerts a slow action upon certain of the substances encountered in 

 passing through the brickwork, thereby producing zeolites. The 

 action is not necessarily attended with solution, but probably in some 

 cases is analogous to the process of cementation. 



The essential elements, therefore, in the production of these min- 

 erals are — circulation of water, and chemical reactions consequent 

 upon such circulation. 



In order to produce crystallized zeolites, it is evident from the 

 facts recorded above that a temperature of 60° or 70° C. is suf- 

 ficient, at least for some of the more abundant species. It seems 

 reasonable therefore to suppose that the formation of these minerals 

 in rocks may take place under the ordinary atmospheric pressure, 

 and at the surface of the ground ; but it is not the less remarkable 



