C. W. Knight — Occurrence of Pseudo-Leucite. 293 



and anorthite all the K 2 may be calculated as present in the 

 orthoclase ; this gives : Orthoclase, 50'04$ ; nephelite, 32*38^ ; 

 anorthite, 2-22^. 



The small amount of lime is considered to belong to the 

 plagioclase, which as previously stated occurs in very subordi- 

 nate quantities. It is thus obvious that the scapolite is com- 

 posed essentially of the marialite molecule (Na 4 Al 3 Si 9 24 Cl). 



The pseudo-leucites from the various localities previously 

 enumerated have generally three common characteristics : (a) 

 the crystal form of the icositetrahedron, (Jj) the a palisade " 

 structure about the border, (c) the similarity of the constit- 

 uent minerals. 



Though the only specimen containing scapolite is that 

 from the Yukon territory, still it seems highly probable that 

 the scapolite in this case is secondary after nephelite and 

 that consequently the original leucite broke down into ortho- 

 clase (with a small amount of plagioclase) and nephelite. The 

 general resemblances just enumerated would appear to indi- 

 cate that all these pseudo-crystals were formed under some- 

 what the same conditions. Just what those conditions were 

 must still be a point open for discussion. It is believed by 

 the writer that the mineral originally crystallized out, in the 

 intra-telluric period of the magma, as a soda- leucite, that after 

 the entire magma had solidified the leucite changed over 

 into orthoclase, nephelite and subordinate plagioclase, and 

 that still later the nephelite ( both in the pseudo-leucite and 

 in the ground-mass of the rock ) changed partly into scapolite. 



Leucite, which under any circumstances is a very unstable 

 compound, is evidently rendered much more so by the large 

 replacement of K 2 by Na 2 0. It would seem that the pres- 

 ence of the latter in such large amount was responsible for 

 the change of the soda-leucite into orthoclase, nephelite and 

 very subordinate plagioclase. 



Geological Laboratory, Columbia University. 



Am. Jour. Sci. — Fourth Series, Vol. XXI, No. 124. — April, 1906. 

 21 



