140 Notes on the Area of Intrusive Rocks at Dargo. 



hills, which are low and rounded as compared with the 

 towering Silurian mountains which surround them. But 

 wherever within this area traces of the zone of contact rocks 

 still remain undenuded, the hills are rough and rugged. 

 The vegetation also invariably tells the observer the 

 character of the rocks, for the diorites decompose into a soil 

 of better quality, clothed with more nutritious grasses, and 

 the forests are less dense, and are of different eucalypts than 

 on the Silurian formation. 



The sample which I selected for examination from the 

 varieties of diorites is of the more basic kind. The other 

 samples strongly resemble similar rocks which I have 

 described from Noyang and Ensay. 



The rock under examination is a crystalline-granular com- 

 pound of medium texture, in which by the pocket-lens 

 plagioclase felspars, dark green to almost black hornblende, 

 traces of black mica, and very little quartz can be made out. 

 The rock is somewhat darker in colour than the generality of 

 the diorites of this neighbourhood. When examined under 

 the microscope it proves to be composed of the following 

 minerals : — 



(a.) A little magnetite, in somewhat larger crystals than 

 are usually found in these rocks. 



(6.) Amphibole, in very cavernous crystals. Some are 

 twinned in the usual manner, and I measured obscuration 

 angles up to 20 deg. Pleochroism is not strong in shades of 

 brown. The mineral has become fibrous, and is also much 

 chloritised. In places small clusters of talc-plates have also 

 resulted from alteration. 



(c.) Triclinic felspars, which predominate in amount some- 

 what over the amphibole. Some of the felspars are broken 

 and crushed, as, indeed, are some crystals of amphibole. The 

 felspars are more or less kaolinised, but in an inequal 

 manner. The size of the felspars crystals differs, some being 

 large and very compound, and having somewhat the 

 appearance of oligoclase. Others are smaller and more 

 simple. The latter are better developed than the former. 

 These differences suggest two generations of felspars. 



In the slices which I prepared there were but few sections 

 of these felspars in which reliable measurements of the 

 obscuration angle could be obtained, and these were all in 

 the plane OP— coPoo . The values thus obtained were from 

 S degs. to 23 degs., thus suggesting a soda-lime felspar on the 

 border between Andesine and Labrador. This, however, 



