BY H. I. JENSEN. 587 



arrived at are sul)ject to doubt and uncertainty. The method in 

 fact becomes painfully laborious and very inexact. When a 

 knowledge of the composition of a rock has been desirable, I 

 have therefore made a complete chemical analysis. 



In my opinion microscopic study and chemical analysis are 

 both equally essential for the correct descri})tion of a new rock. 

 The former reveals, as accessories, minute crystals of fluor, apatite, 

 etc., whose presence indicate traces of F, PoO.,etc., which might 

 easily be overlooked in a chemical analysis. The latter, however, 

 brings out clearly so many facts regarding the affinities of the 

 rock that it cannot be dispensed with. The resemblance between 

 the pantellarites and the comendites of the Glass House Mountains^ 

 and the trachytes of the Warrumbungle and Nandewar Mountains 

 (e.^., W. 16, N. 30, N. 59) is so close that a chemical analysis is 

 essential to feel sure that there is a difference at all. 



The Warrumbungle rocks may be conveniently classified under 

 the following general headings : — 



(a) Arfvedsonite Trachytes, Comendites and Pantellarites, 

 light grey or bluish-grey in colour. 



(b) Dark ^girine Trachytes, Phonolitic Trachytes and 

 Phonolites. 



(c) Trachydolerites. 



(d) Andesitic Rocks and Basalts. 



The mineral contents of the first of these two divisions include 

 felspars, soda-hornblendes, segirine, segirine-acraite, felspathoids 

 like nosean and pseudoleucite, quartz (rare), decomposition- 

 products, and apatite (extremely rare). 



The felspar occurs with three different habits — the orthophyricy 

 as in the Scabby Rock trachyte; the prismatic, ^^ in Timor Rock 

 trachyte; and the tabular, as in the silky trachytes of Gibb's Gap, 

 etc., and the phenocrysts in the dark phonolitic trachytes. The 

 prismatic habit is the commonest, being developed in the majority 

 of the felspars of most of the rocks of these divisions. Tabular 

 felspars are also very common. The orthophyric felspars are 

 confined to the old plugs or necks. The prismatic type occurs 

 both in the trachytes of necks and sheets or Hows. The tabular 



