290 G. W. Knight — Occurrence of Pseudo-Leucite. 



" segregations " which present the aspect of crystals or groups 

 of crystals. These consist of orthoclase, nephelite, augite, 

 hornblende and titanite, with the crystal form of lencite. The 

 marginal layer, rarely absent, is composed of lancet-shaped 

 orthoclase (with nephelite) disposed normally to the faces of 

 the pseudo- crystals. The peculiar masses were studied by 

 Graeff and Hussak ;* the former regarded them as inclusions 

 of pre-existent foyaite ; the latter looked upon them as true 

 pseudo-crystals representing a tendency to the formation of 

 lencite under physical conditions unfavorable to the complete 

 development of that mineral. Derby suggested " that perhaps 

 crystals of leucite exceedingly rich in inclusions (phenocrysts 

 from the surrounding magma) were actually formed; but that 

 before the complete consolidation of the magma, some change 

 of conditions brought about, through magmatic action, a pseu- 

 domorphosis of the leucite molecule into orthoclase and nephe- 

 lite." 



As will be shown below, the pseudo-leucites from Spotted 

 Fawn Creek, Yukon Territory, have about the same chemical 

 composition as those from Magnet Cove, Arkansas, described 

 by J. F. Williams, f At the latter locality they are found in 

 leucite-syenite dikes and leucite-tinguaites. The pseudo-crys- 

 tals occur in icositetrahedrons 5 cm in diameter and consist of 

 a network of allotriomorphically bounded, tabular crystals 

 which are generally arranged in radial forms and which are 

 often interspersed with small allotriomorphic eleolite crystals 

 and idiomorphic pyroxenes. The orthoclase crystals often lie 

 with their symmetry-planes at right angles to the edge of the 

 section and hence forming a sort of palisade about the edge. 

 In the slide of this material examined by the writer it was at 

 once noted that the minerals composing the pseuclo-leucite 

 were coarser in grain than in the pseudo-crystals from the 

 Yukon territory. Wavy extinction was also noted in the crys- 

 tals from Magnet Cove. Williams' hypothesis regarding 

 their formation is similar to that of Derby. He says " they 

 began to form under conditions favorable to the formation of 

 leucite and, as is frequently the case with true leucite crystals, 

 they included all sorts of foreign particles, such as magnetite 

 and aegirite. The conditions then changed and instead of 

 becoming true leucites their substance was recrystallized and 

 the leucite molecule was broken up into eleolite and ortho- 

 clase." 



The pseudo-leucites described by Weed and Pirsson^: from 

 the Bearpaw Mountains, Montana, occur in icositetrahedrons 



*Neues Jahrb. 1887, vol. ii, p. 255; and 1890, vol. i, p. 166. 



f Annual Keport Geol. Sur. Arkansas, 1890, vol. ii, "Igneous Rocks of 

 Arkansas. " 



X This Journal, vol. i, p. 394, 1895, 4th series. See also vol. ii, 1896 (4), 

 p. 194. 



