OCCURRENCE OF TINGUAITE AT KOSCIUSKO, N.S.W. 375 



morphic granules of elaeolite. In another variety the nephelines 

 are small, fairly idiomorphic and enclosed in the felspars, or inter- 

 grown with them so as to form a micropegmatitic structure. In 

 another variety of laurdalite olivine is present in some quantity, 

 so that Brogger applies to the rock the term olivine-laurdalite. 

 While therefore the laurdalite of the type rich in nepheline (type 

 in., Brogger, op. cit., p. 19) differs chemically from the Kosciusko 

 nephelinitic tinguaite in containing less soda but more lime and 

 alumina, it differs distinctly mineralogically (1) in containing 

 much felspar (up to over 62% in the leading type). (2) In con- 

 taining little nepheline, only 13% in the leading type, though no 

 doubt more in variety in., which is described as rich in nepheline. 

 (3) In containing more mica. The Kosciusko rock cannot there- 

 fore be classed with nepheline-laurdalite. As the nepheline-porphry 

 of Beemerville, Sussex Co., N.J. ("Sussexite" of Brogger) is rather 

 too basic to be closely allied with the Kosciusko rock, there remain 

 only the "Phonolitic nephelinites " (Rosenbusch), and certain 

 nepheline-bearing types of tinguaite, to which the Kosciusko rock 

 may be referred. 



The phonolitic nephelinite of Rosenbusch is a trifle more basic 

 than the Kosciusko rock, as already remarked having 2% less 

 silica, about 4% more iron oxides, over 1% more lime, and about 

 2% more magnesia. Unfortunately we have not access to Doelter's 

 paper on the phonolitic nephelinite of S. Antao. The petrological 

 description of it, however, given by Rosenbusch 1 leads us to 

 suppose that it must be very closely allied to the Kosciusko 

 rock, though more basic in composition. As mentioned by 

 Rosenbusch, the rock described by Doelter is remarkable for 

 the absence of augite, the want of coloured minerals as constitu- 

 ents, the abundance of segirine, in the form of microliths, and of 

 sharply idiomorphic nepheline, belonging to two distinct genera- 

 tions. Sanidine occurs sparingly or more abundantly in small 

 laths. The structure is decidedly porphyritic and the rock is 



1 Mikroskopische Physiographie der Massigen Gesteine. — H. Rosenbusch 

 Bd. ii., 2nd Edit, pp. 734-795. 



