188 Notes on the Area of Intrusive Rocks at Bar go 



(1.) This sample was taken from a strong dyke which 

 traverses the quartz diorites on the eastern side of the 

 Dargo River, and it lies as rough masses on the hill-sides, 

 where it crops out. It is compact, and of a greenish colour, 

 weathering to a lighter tint. It has a slightly glassy lustre 

 on a fresh fracture. Under the microscope the ground-mass 

 contains a considerable amount of colourless basis, the 

 remainder being innumerable minute acicular crystals of 

 amphibole, some with ragged ends, and of all sizes, between 

 •0084nch and -OOl-inch in length, and -000 5 -inch and '0001- 

 inch in breadth. 



These crystals lie at all angles across each other in the 

 basis, and are slightly pleochroic. In this ground-mass there 

 are clear and colourless simple crystals of orthoclase. This 

 rock, therefore, is a variety of Syenite. 



(2.) This dyke crosses the gap between Waterford and 

 Dargo. It weathers into rough, reddish-coloured blocks. It 

 is of a rather fine grain, and inclined to purple in tint on a 

 fresh fracture. Eather , numerous small prisms of black 

 hornblende can be observed. In a thin slice I found this 

 rock to have a ground-mass in places of a granophyric 

 structure, but the porphyritic minerals of the rock do not in 

 all cases form the centre of the granophyric masses. These 

 are formed by radially concentric colourless prisms of quartz, 

 which in places include crystals of the other minerals which 

 form the rock. The remainder of the ground-mass contains 

 much colourless basis throughout, in places amounting to 

 rather large patches. 



In this ground-mass are eroded crystals of dark reddish 

 brown mica, which are pleochroic in shades of dark reddish 

 brown and yellow. There are also numerous rather 

 lengthened prisms of amphibole, which in cross sections 

 show the characteristic cleavage of that mineral. The 

 absorption is rather strong, and the colours of the rays are 

 in shades dark chestnut brown, light chestnut red, and 

 yellow. 



(3.) This dyke crosses the quartz diorites at Orr's Creek, 

 and is of a somewhat peculiar character. 



It is black in colour, weathering to greyish-black. In 

 fresh fractures one can observe numerous small shining flakes 

 of pinchbeck to black coloured mica, with pyroxene and some 

 larger isolated felspars. Owing to the extremely dark tint 

 of the ground-mass of this rock, it is most difficult to pre- 

 pare a thin slice in a satisfactory manner. The ground-mass 



