44 GEOLOGICAL EXCURSION TO PORT CYGNET. • 



followintr remarks. Speaking of the porphyry with the 

 large biscuit-like felspar crystals ("magpie' locally), he 

 says : — 



" Large crystals of orthoclase with splendid cleavage 

 parallel to P (001), M (010), and (lOl), a little triclinic 

 felspar, a few pseudomorphs of natrolite after nepheline, 

 pseudomorphs of iron ores after nosean and biotite (?) and 

 some garnets lie in a very fine fluidal trachytic groundmass 

 of felspars. The pseudomorphs after nepheline are the most 

 charming aggregates of natrolite that I have ever seen, 

 except in the tinguaites from Alno. At the same time it is 

 also possible that the original mineral was sodalite. These 

 psendomorphs are also enclosed in the felspars. The rock is 

 strongly impregnated with pyrite. I hesitate only w^iether 

 to call it an alkali syenite porphyry or on elaeolite syenite 

 porphyry." 



The pseudomorphs of iron ore after nosean have been a 

 piizzling question for some time, and I sent a slide to Pro- 

 fessor Rosenbusch containing a section of one of them plaiiily 

 replacing garnet, and suggested that garnet may have been 

 the original mineral in all instances. The Professor's reply 

 shows an openness of mind which is refreshing to meet 

 with now-a-days. It is as follows : — 



" I agree with you in referring the iron ore pseudomorphs 

 in the slide which you sent me to garnet (melanite). At the 

 same time, I am not sure whether this holds good for the 

 pseupomorphs in my slides. It may possibly do so. The 

 diiference in habit may be due to varying thickness of the 

 slices. In my slices the forms remind me of nosean in 

 leucitophyres and allied rocks. Had I seen your slide first, 

 perhaps my interpretation would not have occurred to me." 



It may, therefore, well be that some of these iron ore 

 pseudomorphs are really after nosean, while others replace 

 garnet. Fi'om the fact that we have undoubted nepheline in 

 in this gi'oup of rocks, it is likely that the mati-olitic pseudo- 

 morphs are after nepheline. 



Closely allied with the above is a fresh-looking garneti- 

 ferous porphyr}^ of trachytic aspect, with glistening 

 phenocrysts of sanidine, and spotted in places rather 

 abundantly with small soft grey or white crystals of 

 haiiyne or nosean. Nosean trachyte or phonolite appeared 

 a convenient name, but Professor Rosenbusch call it a 

 melanite-bearing elaeolite syenite porphyry. He writes : — 



"The felspar is fresh and like sanidine. A few small 

 elaeolites are couA^erted for the most part into aggregates of 

 natrolite. The dark constituent is an emphibole with 

 C : 6' = 20° 7 — a weak, ( = /; blue green, ^ greenish yellow. 



