136 Notes on the Area of Intrusive Rocks at Dargo. 



quartz, with a very small amount of mica, which in most 

 •cases appears to be muscovite. In places this alkali-mica is 

 replaced by a brown mica, apparently biotite. Such an 

 example I examined from a vein which crosses the contact 

 near the saddle where the road crosses over to Dargo. A 

 hand specimen is pale buff in tint, fine-grained, and shows 

 small crystalline cleavage planes of felspar and grains of 

 quartz, with very small and rare dark brown flakes of mica. 



This general view is borne out by a microscopic examina- 

 tion. The felspars are for the most part orthoclase in 

 irregularly formed crystals, with a few better crystallised 

 plagioclase crystals. A few small flakes of brown mica 

 scattered about among the grains of quartz complete the 

 compound. 



The cavernous and broken felspars point to alternations of 

 temperature, affecting the partially crystallised magma, and 

 also to the disturbed conditions under which the dykes were 

 forced into opening but resisting sediments. Another sample 

 which I collected near the same place and examined under 

 the microscope showed me orthoclase felspars, intergrown 

 with quartz, after the " graphic " manner of structure. 

 There were also triclinic felspars, much eroded externally. 

 Grains of quartz filled in the interspaces, and a few small 

 flakes of brown mica completed in this instance also the 

 compound. 



In this rock the signs of violence are also clearly to be 

 made out. The felspars have been much broken, and the 

 fragments can be seen jammed into interspaces, thus 

 showing that the rock had been in movement shortly before 

 it had completely consolidated. 



A third example from a dyke-like vein in Orr's Gully is a 

 light-coloured and crystalline-granular rock, in which 

 numerous shining cleavage planes of felspar can be seen; with 

 quartz^ and ver}^ minute and rare plates of black mica. 



As seen under the microscope, it is composed of ortho- 

 clase felspars, in angular or cavernous crystals, which are 

 rather larger relatively, as well as more broken and eroded, 

 than the fewer triclinic felspars which accompany them. 

 Some crystals of microcline also are to be seen. The quartz 

 is in considerable amount. The mica, which is very sparsely 

 scattered through the mass, is brown and fibrous, and only 

 slightly pleochroic. 



Of this sample I carried out a quantitative analysis, the 

 results of which follow : — 



