128 Notes on the Area of Intrusive Rocks at Dargo. 



• — Mount Alfred, Mount Lookout, and Mount Taylor — are 

 capped by Upper Devonian conglomerates and grits. 



These notes refer to part of the Dargo intrusive area. It 

 is divided into two nearly equal parts by the Dargo River, 

 which flows through it in a southerly direction for about 

 six miles. The east and west extent of the area is about 

 twelve miles. It is a comparatively low tract of hills, with 

 smoother outlines than those of the Silurian formations 

 which surround it, and which on the north side rise to a 

 height of over 3000 feet. As in other tracts of similar 

 igneous rocks, the soil is better, and consequently the 

 herbage finer and more fattening than that growing on the 

 surrounding hills. It is to the western part of the Dargo 

 ■area that these notes especially relate. 



Description and Examination of the Rocks. 



No fossils have been discovered in the sedimentary rocks 

 of this district, but there can be little doubt that they are 

 of lower paleozoic age, and most likely Lower Silurian. 

 The least altered examples which I have found are at 

 Waterford, where the road to Dargo crosses the Mitchell 

 River, and, as is elsewhere the case with this formation, 

 the beds are alternations of quartzose sandstones, and 

 argillaceous, somewhat slaty beds tilted at high angles. I 

 selected two samples as being typical, and now describe 

 them : — 



1. Argillite. — This rock is very fine-grained in texture, 

 and of a greyish colour, inclining to a green tint, especially 

 on a cross fracture. It has been afiected by slaty cleavage, 

 which coincides nearly with the planes of deposit. It is 

 faintly wrinkled and slightly shining, on the cleavage planes, 

 with spots and strings of hematite, which lie between the 

 latter. With the pocket-lens very numerous but minute 

 scales of a silvery mica can be made out. 



Thin slices of this rock under the microscope are found to 

 be composed mainly of a colourless mineral in minute flakes, 

 which are mostly arranged parallel with the cleavage. Since 

 the thin slices of this fragile rock can be only prepared 

 (according to my experience) from cleavage pieces, it follows 

 that the plates of the above-mentioned mineral are parallel 

 to the slice. When rotated between crossed nicols the 

 spaces filled by this mineral, even when several plates are 



