690 Z. V. PIRSSON 



commonly intrusive ones, though it is clear special cases might 

 arise where the reverse was the case. Thus Cross ^ is inclined to' 

 view the analcite in phonolites described by him as of primary 

 origin and the latest component to form, its formation being due 

 to the local concentration of aqueous vapor produced by its 

 exclusion where crystallization was progressing and by its accu- 

 mulation in other spots. 



The literature contains abundant references to the occurrence 

 of analcite in igneous rocks, but in the great majority of them 

 the rocks described are altered and unlike the beautifully fresh 

 types whose discussion has served for the basis of this article, 

 and it must therefore be uncertain whether the analcite is of 

 primary or secondary origin. In some cases it would appear to 

 be primary, but a discussion of them would carry us too far. 



It is now clear (whether one accepts the primary origin of 

 analcite or not) that we must recognize the analcite group of 

 rocks just as we have the leucite group. The analcite basalts 

 corresponding to the leucite basalts are the Mo?icIiiquites of Rosen- 

 busch, the analcitites or olivine free analcite basalts are the Four- 

 chites of J. Francis Williams.^ The demonstration that the base 

 of the monchiquite group is not glass but analcite does not in 

 any sense impair its distinctness and individuality as a rock 

 group, on the contrary it strengthens it by giving a definiteness 

 of mineral composition that it did not before possess, and at the 

 same time clears up what was one of the most puzzling questions 

 in the petrography of igneous rocks occurring in these small 

 intrusive masses. 



And in conclusion we note also that the present case presents 

 another example of how magmas, possessing similar chemical 

 compositions, may form different mineralogical products when 

 crystallizing under different physical conditions. 



L. V. PiRSSON. 



MiNERALOGICAL-PETROGRAPHICAI. LABORATORY, 



Sheffield Scientific School, Yale University, 

 New Haven, May 1896. 



'Geology Cripple Creek, XVI, Ann. Rep. U. S. G. S., Ft. II, p. 36, 1895. 

 ^Igneous Rocks, Ark., p. IIO, 1890. 



