366 T. W. E. DAVID, F. B. GUTHRIE, AND W. G. WOOLNOUGH. 



more or less kaolinised and much corroded fragments of felspar with an 

 occasional granule of quartz, both being evidently derived from rocks 

 intruded by the basalt dyke, so that they may be considered to be en- 

 closures. A granular greenish-yellow epidote is present with a pleochroism 

 in grey to deep greenish-yellow. It probably represents an intermediate 

 stage of alteration between the colourless augite and the chlorite. In 

 one slide the basalt contained a small enclosure of kaolinised felspar with 

 a flake of biotite intercrystallised with it. This enclosure was obviously 

 derived from the local granite. 



IV. Chemical Composition. — The chemical nature of the rock 

 is shown in the accompanying table which exhibits the result of 

 analyses (made by one of the authors, Mr. Guthrie), of the three 

 specimens taken from different portions of the dyke, the micro- 

 scopic characteristics of which have been just described. Specimen 

 No. I. was taken from the northern side of the dyke, and represents 

 a thickness of the dyke of six inches, measured at right angles 

 to the plane of junction of the dyke rock with the granite. No. 

 IV. represents a similar portion of the dyke on its southern side, 

 and No. III. was taken from the centre of the dyke. As already 

 stated, the dyke is about seven feet wide. 



Composition of nephelinitic tinguaite from Kosciusko. 







I. 



III. 



IV. 



Water 



at 100° C. 



0-23 



0-78 



0-32 



Water above 100° C. 354 



5-25 



3-29 



Si0 2 





5240 



5015 



51-98 



A1A 





19-93 



1845 



20-61 



Fe 2 3 





3-83 



4-71 



4-08 



FeO 





1-51 



1-24 



1-32 



MnO 



... ... 



0-45 



0-26 



0-40 



CaO 





1-34 



1-39 



1-12 



MgO 





0-32 



0-37 



0-38 



K 2 





4-10 



4-65 



4-42 



Na 2 



... ... 



11-71 



12 02 



11-69 



so 3 





none 



none 



none 



CI 





0-05 



traces 



009 



PA 





traces 



traces 



traces 



co 2 





0-21 



0-52 



0-44 







99-62 



99-79 



100-14 



Oxygen equivalent for CI 001 





•02 







99-61 





10012 



Specific Gravity 



2-499 



2-434 



2-492 



Decompos 



able by HC1 



66-0 



60-0 



66-0 



