156 Ernest W. Skeats : 



ally to the close of this period (Permo-Carboniferous). It is defi- 

 nitely known that they are intrusive into the Lower Marine 

 sandstones and mudstones, and they appear to be cut through by 

 the diabase which is considered to date from the close of the 

 Mesozoic. The belt comprises the following rock varieties : — 



Alkali Syenites. — Quartz augite syenite, Aegirine augite syenite. 

 Alkali syenite porphyry. 



Elaeolite Syenites. — Pyroxene foyaite, Mica foyaite, Jacupir- 

 angite, Amphibole foyaite porphyry, Solvsbergite porphyry, Mica 

 solvsbergite, Tinguaite porphyry, IMonchiquite nephelinite. 



Essexite. — Essexite. 



Auriferous quartz and pyrites have lieen developed near the 

 line of contact of these igneous rocks with the Permo-Carboniferous 

 sediments, and a good deal of alluvial gold has laeen recovered from 

 the creeks and flats. 



(9) Dr. H. I. Jensen, in Proc. Linn. Soc, N.S.W., 1908, pp. 

 557-558, referring to tlie rocks of the Port Cygnet group, remarked 

 on their general close resemblance to Australian alkaline rocks, and 

 stated that they were considered without very much evidence to l>e 

 of Lower Mesozoic age. They are known to be later than the Permo- 

 Carboniferous, and to antedate the Pliocene, but direct evidence to 

 fix their age more closely appears to be wanting. 



(10) Professor David and the Avriter wrote a chapter on the 

 igneous rocks in the Geology of the Commonwealth in the Federal 

 Handbook on Australia for the British Association meeting of 1914. 

 On p. 309, under the heading, Jurassic (?) (possibly Triassic), they 

 refer to the foyaitic rocks of the Port Cygnet series. These rocks 

 are considered to be perhaps of Lower Mesozoic age. They are all 

 strongly intrusive into the Permo-Carboniferous series, but their 

 relations to the Jurassic sedimentary rocks and to the diabase have 

 not yet been clearly demonstrated. 



Distribution of the Alkali Rocks. 



Two groups of outcrops of alkali rocks occur in this district,! one 

 in the S.W. of the area on the shores of the Huon Eiver and of 

 Port Cygnet, the other in the N.E. part of the area, on the shores 

 of Peppermint Bay and Little Oyster Cove on D'Entrecasteaux 

 Channel. The exposures of alkali rocks in the first group in the 

 S.W. of the area include the following : — An outcrop in Petchey's 

 Bay on the Huon River, dykes of porphyry at Lymington, a quarter 



1 See locality iiiup. 



