NEPHELIXE IN PHONOLITE DYK!:S. 127 



Gippsland. This collection and a sketch geological map of the 

 area were made by the late Mr. James Stirling in 1884, and 

 sent to Dr. Howitt for examination. Beyond having some rock 

 sections made. Dr. Howitt does not appear to have done any further 

 work with these rocks. This communication deals with a few of 

 these dykes. 



It is to be noted that the nearest nephehne bearing dykes to 

 those about to be described occur at Kosciusko, in New South 

 Wales. They were described by Professor David, Dr. Woolnough 

 and Mr. Guthrie in the Proceedings of the Royal Society of New 

 South Wales, 1901, p. 347. They are described as tinguaites, and 

 besides being more basic than the Omeo rocks present other points 

 of difference. 



Physical Geography. — The area lies north and south of the 

 Omeo township, in Eastern Gippsland. Between Wilson's and 

 Day's Creeks, two confluents of the Livingstone Creek, there rises a 

 somewhat abrupt hill to the height of about 500 feet above the levels 

 of the streams. This is locally known as Day's Hill or Frenchman's 

 Hill. The township is built on undulating ground, which continues 

 to the south, and it is in this area that the numerous dykes of the 

 district chiefly occur. (See map, Plate IX.) 



Geology of the Area. — Howitt has given a description of the 

 district.^ A brief summary of the geological relations was given by 

 myself in a presidential address to Section C at the Brisbane meeting 

 of the Australasian Association for the Advancement of Science in 

 1909. 



The oldest rocks lie to the east of the area under consideration, 

 and consist of crystalline schists and gneisses of unknown age. 

 They are probably pre-Ordovician and possibly pre-Cambrian. 

 Intrusive into these are acid granites and aplites and also quartz 

 diorites. Surrounding these plutonic masses are dykes of varying 

 character, ranging from older to younger, as follows : — 



1. Orthoclase, muscovite quartz. 



2. Orthoclase, quartz. 



3. Quartz, orthoclase, schorl. 



4. Quartz with a little mica. 



5. Quartz. 



In Howitt's map he shows a north and south fault bringing 

 the plutonic rocks abruptly against the older schists. 



Younger than these plutonic rocks with their associated dykes 

 are the intrusive rocks of Frenchman's Hill, described by Howitt as 

 orthophyres consisting mainly of orthoclase with a little soda- 

 augite. I have shown {op. cit.) that the felspar is principally 

 anorthoclase, that most of the rocks are better described as anortho- 

 clase trachytes and solvsbergites, and that in the central part of the 

 hill, where the rocks approach a plutonic habit, a little interstitial 

 quartz occurs. Radiating from Frenchman's Hill are a series of 

 trachytic dykes, some of which are shown on Howitt's map. and 



1 Howitt, A. W. : " On Certain Plutonic and Metainorphic Rocks at On.eo," Rep. Min. Dept. Vict., 

 March, 1890, p.p. .H2-4(i. I'lafes an.l Maps. 



