Analcite-Trachyte Tuffs and Breccias. 273 



showers of volcanic debris simultaneously with the other 

 materials of the tuff, and was therefore a primary constituent 

 of the partially consolidated magma which gave rise to these 

 rocks. (1) The analcite almost invariably occurs in crystal 

 fragments or sometimes in nearly perfectly preserved crystals 

 (icositetrahedrons). (2) Though the series of specimens 

 studied are not on the whole fresh, still it is common to find 

 perfectly fresh orthoclase and aegirite closely associated with 

 analcite. This we would not expect to find if analcite were 

 secondary after leucite (as was at one time commonly supposed) 

 because such a radical change in chemical composition could 

 not take place without effecting the orthoclase. (3) The fact 

 that the various minerals sometimes reach a diameter of an 

 inch and a half shows that crystallization took place, partly at 

 least, at considerable depths and therefore under sufficient 

 pressure to retain the necessary water in order that analcite 

 should form. (4) A very insignificant amount of what is 

 undoubtedly secondary analcite does occur, sometimes partly 

 replacing aegirite and sometimes occuring in microscopic 

 veinlets. This material is quite clear and colorless, and differs 

 in this respect from the primary analcite. 



It is possible that leucite in very small amount also occurs 

 in the series of rocks, for in the ground mass of a rock fragment 

 in a breccia were found roundish, clear, isotropic individuals 

 .005 mm. in diameter. The minuteness of the material made 

 separation of these isotropic crystals impossible; micro-chemical 

 tests, however, were made on the very fine grained ground mass 

 containing these crystals, giving reactions for potash but not for 

 soda, proving the absence of analcite. The potash reaction 

 may have resulted from the orthoclase which also occurs 

 associated with these crystals, so that it cannot be definitely 

 stated that leucite is present. 



The Rock Types Found. 



There are four rock types which may be distinguished in 

 the specimens examined. About sixty thin sections were studied 

 under the microscope. With one exception all the sections are 

 seen to be characterized by high per cents of soda and potash, 

 while quartz is persistently absent in all of them. 



