ITALIAN PETROLOGICAL SKETCHES 843 



occur. They are all of a dull pure white and extremely friable 

 — due to a process of kaolinization. A few small augite pheno- 

 crysts are also visible. 



Under the microscope they resemble somewhat those just 

 described, the differences being largely due to decomposition. 

 The large leucites, many of which have fallen out, are of a pale 

 yellow color and quite isotropic. The original leucite substance 

 has given place to a peculiar alteration product which here and 

 there is subfibrous in structure, but more often resembles a gum. 

 It is filled with perlitic cracks which split it up into spheroidal 

 masses. 



The groundmass is very fine-grained and rather hyalopilitic 

 in structure, formed of a felt of small diopside and alkali feld- 

 spar laths with magnetite grains, lying in a base either of 

 brownish glass or glass with flakes of orthoclase. Scattered 

 through it are small leucites, from 0.02 to Co;""", which, though 

 perfectl}' isotropic, leave absolutely no doubt as to their real 

 nature, since they show the characteristic isolated inclusions, 

 and often the still more characteristic skeleton forms previously 

 described. 



The leucites in these rocks, as well as in some of the flows, 

 are supposed by vom Rath, Bucca, and Deecke to be inclosures 

 derived from leucitic lavas of Monte Vico which were caught up 

 during the eruption of Ciminian " trachytes." This view is 

 based on the supposed facts that the leucite is always pheno- 

 crystic, is always or generally in fragments, is always kaolinized, 

 and also on the chemical composition of the rock. 



With this theory I cannot agree. In the first place my own 

 observations lead me to agree with Verri (p. 29) in thinking 

 that the greater part, if not all, of this petrisco-bearing tuff was 

 derived from the Vico crater. Furthermore, the leucite, while 

 chiefly present as phenocrysts, also occurs abundantly in the 

 groundmass as we have seen. The fragmentary character of the 

 large leucites did not appear to me to be nearly as universal as 

 stated b\- the above writers ; and even were it so it would not be 

 surprising when the peculiar molecular condition of the mineral is 



