374 T. W. E. DAVID, F. B. GUTHRIE, AND W. G. W00LN0UGH, 



Brogger calculates (op. cit., p. 161) the mineral constitution of 



is nepheline 



porphyry to be as follows 

 40J Alkali-felspar 

 321 Nepheline 

 11 Sodalite 





8 

 5 



2 



Pyroxene 

 Lepidomelane 

 Iron oxides 





1 Sphene 

 \ Apatite 



100 



The nepheline porphyry therefore of the Lougenthal differs 

 from the Kosciusko nepheline rock in being (1) much more coarsely 

 crystalline, being eugranitic in texture, in which respect as 

 Brogger remarks, it approaches the nepheline syenites to which 

 he has applied the name of ditroite. (2) In containing some 

 phenocrysts of felspar. (3) In containing more sodalite. (4) In 

 containing more mica. (5) In containing sphene, which does 

 not appear to be present in the Kosciusko rock. (6) In contain- 

 ing less nepheline than the Kosciusko rock. 



A remark of Brogger with reference to this nepheline-porphyry is so 

 applicable to the Kosciusko rock, and bears so directly on the question 

 as to whether the Kosciusko rock can truly be termed porphyritic or not, 

 that we think it well to quote it here (op. cit., p. 157) : — 



" Wenn oben, wie in friiheren Publicationen, das Gestein als Nephelin- 

 porphyr bezeichnet ist, muss ich hier eigentlich eine Reservation ein- 

 schalten, Es is namlich offenbar, dass die scheinbaren grossen Einspreng- 

 linge Keine Scharf getrennte dltere Generation im Vergleich mit den Nephel- 

 inkorner der Grundmasse darstellen. Sowohl die scheinbaren Einsprenglinge 

 als die Nephelinkorner der letzteren sind namlich gewiss im wesentlichen 

 gleichzeitig gebildet." 



Also (op. cit., p. 157) "Uberhaupt is diese structur, welche ganz analog 

 auch bei dem Laurdalit selbst wiederkehrt, keine echte Porphyrstructur 

 mit zwei getrennten Generationen, Sondern nur eine porphyrahnliche 

 eugranitische Structur." The italics in the above two quotations are ours. 



As regards laurdalite (Brogger) this rock is characterised in its 

 type localities (Brogger, op. cit., p. 15) by large hypidiomorphic 

 felspars having a subparallel arrangement and large hypidio- 



