C. W. Knight — Occurrence of Pseudo-Leucite. 287 



from the Bearpaw mountains in Montana, described by Weed 

 and Pirsson.* 



Thin sections from specimens A and B when examined 

 under the microscope showed the rock to be made up of 

 the following minerals : orthoclase, pseudo-leucite, nephelite, 

 scapolite and biotite. The first three minerals play the part of 

 phenocrysts; orthoclase and nephelite also appear in the ground- 

 mass and consequently occur in two generations. 



Orthoclase: — The ground-mass consists largely of orthoclase 

 laths, showing flow structures. They exhibit a tendency to 

 flow around the pseudo-leucite. Wavy extinction is always 

 observed with crossed nicols. These laths average about 'l mni 



Fig. 1. Pseudo-leucite crystals from Spotted Fawn Creek, Yukon Tei*ri- 

 tory, Can. The crystal form (ico^itetrahedron) is that of the mineral leucite. 

 Diameter of large pseudo-crystal is l cm . The crystals have been very 

 slightly flattened by pressure. 



in length, while the phenocrysts, which often show the com- 

 mon Carlsbad twins, average 5 mm in diameter. 



Pseudo-leucite : — These extremely interesting and puzzling 

 psendomophs consist of the following minerals : orthoclase, 

 scapolite, nephelite, biotite and a very little plagioclase. The 

 orthoclase is most abundant and occurs under two conditions: 

 first, in allotriomorphic grains, less than 'l mm in diameter, 

 generally concentrated in the more central portions of the 

 pseudo-crystals ; second, in lath-shaped individuals often con- 

 centrated on the outer borders and oriented in such a way that 

 their longer axes lie normal to the crystal faces of the pseudo- 

 leucite (see fig. 2). -It is a striking fact that this border 

 arrangement appears to be a characteristic of the pseudo-leucites 

 from Brazil, Magnet Cove, Arkansas, and the Bearpaw Moun- 

 tains, Montana. Williams in his description of pseudo-leucites 

 from Magnet Cove has referred to this arrangement of the 

 orthoclase as a "palisade" structure. f The allotriomorphic 



* On some Phonolitic Eocks from Montana, this Journal, vol. i, p. 394, 1895, 

 see also vol. ii, Aug. and Sept. 1896, p. 194. 



f The Igneous Eocks of Arkansas— Annual Eeport Geol. Sur. Arkansas, 

 vol. ii, 1890, p. 268. 



