Age of Metamorphic Rocks, Victoria. 129' 



muscovite which are crumpled and are clearly allothigenous. 

 The slide includes one thick layer of iron stained argillaceous 

 material, in which the quartz grains are small and scarce. The 

 clastic origin of most of the. material is apparent. 



In the spur at the western side of Twist Creek occurs a coarse 

 grit [56 ; PI. XX., Fig. 2] with large quartz grains, nearly all of 

 which are rounded. There are also some rounded grains of horn- 

 blende. The interspaces between the quartz grains are occupied 

 by a fine grained quartz mosaic, due to secondary crystallization. 

 Owing to the crushing of the rock, the quartz grains have crushed 

 and caused a certain flow in the softer material. The rock can, 

 therefore, be described as a mylonitic quartz grit. Another 

 variety of arenaceous sedimentary rock occurs a little up Twist 

 Creek, beyond the spur from which the last specimen was 

 obtained. This rock [54; PI. XXI., Fig. 4] is a fine-grained grit; it 

 is composed of quartz, plagioclase and muscovite. The grains are 

 irregularly arranged; the material is all allothigenous; and the 

 rock represents a slightly altered sandstone, of which the materials 

 were derived from some igneous rocks. A similar rock [58 ; 

 PI. XXI., Fig. 3] from the bed of Twist Creek, near the junction 

 of the orduvician with the metamorphic .series, shows the same 

 materials ; it is a coarser grit in which the fragments are often 

 irregularly oblong, and they are surrounded by tine material in 

 curved lines. The material is stained brown by iron oxide. 

 Between these grains are lines of quartz mosaic, and some of the 

 larger quartz grains are beginning to show alteration into mosaic; 

 but this structure is only developed on the edges or along lines 

 running through the grains. 



In the bed of Twist Creek, near the junction with Nine Mile 

 Creek, is a rock [64] showing a further stage in the develop- 

 ment of the quartz mosaic. Examined under singly polarized 

 light the rock appears to consist of bands of white structureless 

 material, separated by layers composed of colourless rounded 

 grains, strongly set in a base of pale green slightly pleochroic 

 material. On further examination the green material is shown 

 to be mostly chlorite; the rounded grains in it are quartz and 

 plagioclase, and the white bands break up into quartz mosaic, no 

 doubt due to secondary change, as the large quartz grains are also 

 passing into mosaic. 



