1907-1908.] 97 



The interpretation of these facts would be a difficult problem 

 to those Mineralogists who regard Zeolites as decomposition 

 products, either of epigenetic or solfataric origin, 1 but without 

 disregarding the latter modes of genesis, which can be traced only 

 in isolated cases, it is quite evident, from a careful study of these 

 Co. Antrim specimens, that the Zeolites in general are for the 

 most part deposited during the final stages of the cooling of 

 the lava, in a process that is purely constructive. It is probable 

 that the presence of Zeolites is dependent upon the amount of 

 natural water in the original magma. As the latter cools and the 

 primary minerals of the rock crystallize, the more liquid, hydrous 

 portion of the magma is left in the interstices of the partially 

 crystallized mass. To a great extent this hydrous portion of the 

 magma, which I have called the residual magma, makes room for 

 itself — the contractile power of the cooling magma being largely 

 due to the presence of water — assuming all kinds of shapes from 

 minute interstitial druses to proper cavities more or less spherical, 

 in which latter case is evidenced the expansive power of steam and 

 dissolved gases. From this hydrous residual magma, which is 

 closely related in composition to the combined average com- 

 position of the primary minerals, except for the presence of water, 

 crystallizes the hydrous glass followed by Natrolite and Analcime 

 and other Zeolites in sequence. The formation of the hydrous 

 green glass may be regarded as the intermediate stage between the 

 crystallization of the primary and the " secondary " minerals. The 

 formation of this glassy layer separates the primary minerals from 

 the residual magma, but, when this glassy material is sparse, or 

 when it is brecciated, the primary minerals float into the hydrous 

 magma, which, containing all the materials necessary, continues 



i. Vide]. G. Goodchild's " Natural History of Scottish Zeolites," Page 

 5 (Published 1903 by Geol. Soc. of Glasgow) ; also Prof. Mier's "Introduction 

 to Mineralogy," p. 483 ; also Prof. Judd's " Student's Lyell," pp. 458-9, p. 484, 

 etc. ; also Prof. Credner's " Elemente der Geologie," pp. 105-6 (1902); also 

 other modern text-books. 



