186 president's address — section c. 



Howitt experienced the same difficulty with these rocks as ;with. 

 those from Mount Leinster in reconciling the identification of th^ 

 felspar as orthoclase with the high percentage of soda in the analysis. 

 The explanation is, as before, that much of the felspar is anortho- 

 clase, ' and a sodarhornblende is also present in some of the rock 

 sections. 



Some of the felspars are rectangular in outline, giving a typical 

 orthophyric structure to the rock. In others flow structure is 

 prominent and the habit tj-pically trachytic. In such casies the rock 

 is best described as an alkali-trachyte (Analysis I., for example). Some 

 of the intrusive material is allied in structure and composition to the 

 solvsbergites (Analysis 2). The latter more acid dykes contain a little 

 free quartz, sometimes granular, sometimes in graphic intergrowth 

 with felspar. One of the radial dykes examined (Analysis 3) is minera- 

 logically and chemically in agreement with the tj'pical bostonites. 



The field evidence combined with microscopic and analytical 

 examination shows that the later intrusions from this alkali centre 

 became progressively more acid in character. 



The Rocks of Noyang, in Dargo. 

 Geographical and Geological Relations. 

 The area over which the alkali rocks occur lies to the east of 

 the Tambo River, with the exception that above the Noyang Ford, 

 now known as Tambo Crossing, a big offshoot from the main mass 

 extends from near Mount Elizabeth to the westward of the river, and 

 along the course of the Haunted Stream. The great bulk of Mount 

 Elizabeth, rising to nearly -3,000 ft. above sea level, forms the central 

 mass of the rocks to be described, and from it many prominent dykes 

 extend in a radial manner. As described by Howitt (52), older Palae- 

 ozoic sediments comprise the fundamental rock of the district, into 

 which were intruded a succession of igneous rocks, at first plutonic 

 quartz-mica-diorites, and succeeding these but genetically related to 

 them was a succession of intrusive and volcanic rocks of progressively 

 more acid composition. These are described by Howitt as quartz-mica- 

 porphyrites and quartz granophyrites (spherulitic quartz-porphyrites), 

 succeeded by quartz-porphyrites, and these by quartz-felsophyrites. 

 Later dykes of diorite and diabase were intruded, and are regarded 

 by Howitt as possibly belonging to the same geological period, 

 although the evidence is slender. The alkali rocks of this area com- 

 prise the following: — (1) The qi^artz-porphyrites and quartz-grano- 

 phyidtes, which occur as dykes and masses penetrating the quartz- 

 mica-diorite. (2) The quartz-porphyrites which occur as milk-white 

 close-grained masses and dykes representing the magma freed from 

 its basic elements and consisting of felspar and quartz alone. (3) The 

 quartz-felsophyrites may also be of alkaline character, although no 

 analyses have been made of this rock, and few minerals are developed. 

 It occurs as a volcanic focus, shows well-defined flow structure, and is 

 flanked by breccias consisting of the altered sediments of the district. 

 It probably represents the plug of the vent from which the earlier 

 intrusions radiated. Many of these observations I have had the 

 opportunity of verifying in the field under the guidance of the late 

 Dr. Howitt. 



