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



with other occurrences, two crystals were selected for analysis. 

 The crystals, which were of a medium gray color with two 

 or three white streaks running through them, broke away 

 very readily from the ground-mass. This fact seems to in- 

 dicate that the original leucite on breaking up into ortho- 

 clase, nephelite and plagioclase contracted so that it could 

 be isolated easily from the ground-mass. It also goes to 

 show that the breaking down of leucite probably occurred 

 when the rock had entirely solidified, but was still at a 

 temperature not far below its fusion point. Fig. 1 is a photo- 

 graph of the two crystals analyzed; their actual diameters 

 are about l cm . 



A. B. c. 



Spotted Fawn Creek, Magnet Cove, 



Yukon Territory. Arkansas. Mt. Vesuvius. 



SiO„ 58-15 55-06 58-30 



A1.0. 23-66 



FeA 1-69-' 25 ' 26 23 ' 8 ° 



CaO 0-43 0-60 0-96 



MgO 0-21 0-28 0-17 



]S T a 2 708 7-60 5-80 



K„0 8-49 10-34 10-94 



II 2 "0- o-io 



H„0+ 1-25 1-78 (ignition) 



CI - trace* 



100-96 100-92 99-97 



* Present, but not determined. 



A. Spotted Fawn Creek, Yukon territory, Canada; analyzed 

 by C. W. Knight. 



B. Magnet Cove, Arkansas, analyzed by J. F. Williams. 



C. Mt. Vesuvius, analyzed by Ramelsberg. 



For comparison two other analyses are given in the above 

 table. The close resemblance which the pseudo-leucites from 

 Magnet Cove bear to those from the Yukon territory is at 

 once noted. From the high amounts of Na„0 which are 

 shown by the above analyses it seems clear in each case that 

 the original mineral was a soda-leucite. On the presumption 

 that the scapolite in the pseudo-crystals from the Yukon terri- 

 tory is secondary after nephelite and that consequently the 

 leucite broke clown into feldspar and nephelite, we may find 

 the proportions in which these minerals occurred by recasting 

 the analysis. The biotite may be considered as inclusions (or 

 as a secondary product from inclusions) in the original leucite. 

 Since only a small per cent of biotite is present, the amount of 

 K 2 required for this mineral is not great. To obtain, there- 

 fore, an idea of the relative amounts of orthoclase, nephelite 



