178 Washington — Igneous Rooks from Eastern Siberia. 



la. Ilia. 



Orthoclase 55*5 Orthoclase. .' 47*4 



Albite 12*5 Anorthite ._ 9*0 



Nephelite 20*3 Leucite 12*7 



Extra alumina 3-0 Nephelite 18*0 



Hornblende 5*2 Aegirite „_._._- __ 6'9 



Biotite 3-4 Augite 6*0 



Zircon 0*1 



100-0 100-0 



The calculation of Ilia is somewhat arbitrary, and is based 

 on the assumption that all the soda, above that required for 

 aegirite, goes into the nephelite molecule. Presumably some 

 of it is in the feldspar, as albite, which would increase the 

 actual amount of leucite. Judging from the sections this is 

 very probably the case, but the relative quantities in each are 

 quite unknown, so the simplest assumption is made above. 

 The anorthite undoubtedly exists in the fine-grained ground- 

 mass, as its occurrence with orthoclase is a prominent feature 

 of the Italian volcanic rocks. It is, however, difficult to 

 detect it, and the augite may be somewhat more calciferous 

 than the usual run of Italian augites, i. e. richer in the diopside 

 molecule. Its amount as calculated is also increased by the 

 assumption, made as usual, that all the Fe 2 3 is in acmite. 



In both cases the quantities of dark minerals calculated cor- 

 respond well with the appearance in thin section. 



From a general point of view this occurrence of foyaite is 

 of interest, since heretofore these rocks have been observed in 

 Siberia, and indeed in Asia outside of India, only at the Ilmen 

 Mountains (Miask) in the extreme western part, this occur- 

 rence being long known. Recently another locality has been 

 described by Morozewitch,* at Marinpol, Gov. Ekaterinoslav, 

 in southern Russia, which may possibly, indeed probably, 

 belong to the same general petrographical province. 



The foyaites of Miask differ from that of East Cape in 

 being in general richer in soda and poorer in potash, resem- 

 bling in this respect the normal types. Analyses of them are 

 given in the Guide to the Excursions of the Yllth International 

 Congress (No. Y, A. Karpinsky, p. 22, 1897), but as the 

 analyses differ much among themselves, and are apparently not 

 very reliable, it seems scarcely worth while to quote them. 



The Marinpol nephelite-syenites are still higher in soda, one 

 of them giving a ratio of K 2 : Na 2 = 1 : 24, though it is not 

 stated in the reference whether this is molecular or not, nor is 

 the analysis given. It may be mentioned that to this pre- 



* J. A. Morozewitcb, Ref. in Neues Jahrb. Centralblatt, 1901, p. 121. 



