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



Art. XXIII. — A New Occurrence of Pseudo-Leucite;* by 

 C. W. Knight. 



Through the kindness of Dr. A. E. Barlow of the Canadian 

 Geological Survey there came into the possession of the writer 

 two rock specimens whose characters are herewith described. 

 Mr. R. G. McConnell of this Survey collected the material 

 during the summer of 1904 and has sent this note regarding 

 the field relations : 



" The leucite rock described in the following paper occurs 

 in the Ogilvie range on the upper part of Spotted Fawn creek, 

 a tributary of Twelve-mile river, Yukon Territory. It occurs 

 in a long dyke-like area about a mile wide where crossed. It 

 is bordered on the north by a reddish granitoid rock, probably 

 a syenite, and on the south by altered sedimentaries, mostly 

 quartzites and slates of Upper Palaeozoic age. It has suffered 

 considerable deformation, the leucite crystals in places being 

 crushed into lines. 



" The leucite rock is probably intrusive rather than effusive 

 in character, although the examination was too hurried to 

 obtain definite information on this point. It appeared to be 

 older than the syenite and younger than the sedimentaries." 



The work was carried on in the geological laboratory of 

 Columbia University, ~New York, and the writer would express 

 his acknowledgments to Professor Kemp and Dr. Berkey 

 for advice and assistance. 



Specimen A is of a medium gray color and perfectly fresh. 

 The ground-mass is very fine-grained and the constituent min- 

 erals cannot be distinguished with the naked eye. The prin- 

 cipal phenocryst occurs in well developed icositetrahedrons 

 usually less than l cm in diameter. Fig. 1 is a photograph 

 of two of these phenocrysts showing their crystal form. In 

 color they resemble the ground-mass of the rock so as not to be 

 distinguished from it. They, however, are readily separated 

 from the matrix. The crystal habit is that of the mineral leu- 

 cite, though in the microscopical examination given later on 

 no trace of an isometric mineral was found. A few sanidines 

 tabular in form and generally less than 5 mm in diameter are also 

 seen to play the part of phenocrysts. 



Specimen B is quite similar to the one just described with 

 the exception that it lias not as many of the icositetrahedral 

 crystals. Instead are seen grayish white, rounded areas 5 mm in 

 diameter with apparently no crystal form. Both specimens 

 show a striking resemblance to the pseudo-leucite tinguaite 



* Published by permission of the acting Director of the Canadian Geolog- 

 ical Survey. 



