ALKALI ROCKS OF DUNDAS, VIC. 133 



the rock. Other rocks in the Coleraine area are also described as 

 trachytes. 



In 1901 Mr. Dennant^ contributed further notes on the trachytic 

 rocks of this area. He confirmed Mr. Hogg's determination of the 

 rocks as trachyte, and also gives chemical compositions of the rock 

 and of the porphyritic felspars. 



At the last meeting of this Association, Professor Skeats^ in 

 his presidential address briefly described some of the rocks of this 

 area, the rock from the quarry at Coleraine being described as an 

 anorthociase trachyte, consisting essentially of orthophyric prisms 

 of anorthociase, with some asgirine, partially or wholly changed to 

 chlorite. 



Physiographic Features. — The area in which the alkali rocks 

 occur extends from the neighbourhood of Coleraine westward to 

 Casterton, and northward to the Glenelg River. This area may be 

 described as forming part of the extensive coastal plain which 

 occupies a large portion of the extreme west of Victoria. The more 

 elevated portions are perfectly flat-topped, and are capped by 

 Tertiary deposits. In general this plain has been deeply dissected 

 by numerous streams, and it is only along the courses of these 

 creeks that outcrops of early Tertiary and Pre-tertiary rocks are 

 exposed. 



From a general survey of the country it is evident that before 

 the formation of these comparatively youthful valleys little, if any, 

 rocks other than the late tertiary deposits were exposed at the 

 surface. As, in addition, the whole area is generally covered with a 

 deep mantle of soil, and the floors of the valleys consist of deep 

 aUuvium, it follows that any attempt to work out the relationship 

 of the various rocks in the district is attended with the utmost 

 difficulty. 



One very noticeable feature near Coleraine is the number of 

 deep " cut-aways " which are present in the minor valleys leading 

 to the main stream courses of the district. These cut-aways have 

 gorge-like courses up to 15 or 20 feet in depth, and generally expose 

 a corresponding thickness of alluvium, although the underlying 

 rocks may outcrop along the floor. Most of these cuts are of fairly 

 recent origin, and the number is increasing every year. At first 

 sight it would appear that the cause of the activity of these small 

 rivulets was that they had been rejuvenated owing to recent up- 

 lift. A more probable explanation, however, is that the valley 

 floors were formerly protected from denudation by the thick crop 

 of reeds and grass which then covered the surface of the ground. 

 Owing to the stocking up of the country with sheep the reeds and 

 grass have been thoroughly eaten down, and the running water has 

 readily dissected the soft alluvium now shorn of its protection. 

 Mounts Adam and Eve were undoubtedly formerly completely 

 buried under Tertiary deposits, but owing to the dissection of this 

 portion of the area by the Konong-Wootong Creek and its tributaries 



1 Proc. Roy. Soc. Vict., Vol. XIV. (N.S.), Pt. I., 1901, p. 10. 



2 Op. cit., p. 21)7. 



