132 Notes on the Area of Intrusive Rocks at Dargo. 



(2.) This sample was obtained somewhat nearer to the 

 contact than sample No. 1. It is grey in colour, and the planes 

 of separation show small spots of slightly different tint, 

 which are still clearer on weathered surfaces. A thin slice 

 is found when examined under the microscope to be made 

 up of a completely felted mass of minute flakes of colourless 

 mica, with rather larger scattered flakes of brown mica. 

 There are .also a few scattered flakes of what appears to be 

 graphite. The "spots" are lighter in colour than the rest of 

 the rock, of much the same composition, but with less brown 

 mica and graphite. There are no quartz grains. 



(3.) This sample was collected still nearer to the contact, 

 and about midway between it and sample No. 1. Its micro- 

 scopic appearance resembles that of No. 2, but with a 

 less flne texture. Under the microscope the structure is 

 as in that sample, but the plates of mica are larger. 



The next samples, at about a quarter of a mile from the 

 contact, were schistose hornfels. The changes which I have 

 noted are very much those which have been observed and 

 recorded elsewhere in rocks of the same class. There is a 

 gradual and more complete conversion of the argillaceous 

 material of the rocks into mica, and as the hornfels is 

 approached, an increase of silica, together with a final 

 complete recrystallisation of the rock. 



The hornfels rocks of this locality difler but little from 

 those which I have described from other places and in 

 former papers. The least altered rocks are those in which 

 the outward general appearance of the sedimentary bed is 

 still retained. In the most altered examples the bedding is 

 almost obliterated, and is only clearly recognisable when the 

 rocks are looked at in mass in situ. 



A very common type of hornfels is plentiful in Orr's 

 Gully. A close-grained, crystalline, dark blue, or purple, 

 or almost black rock, which in the stream-beds, where the 

 rocks have been laid bare, can be seen to be distinctly 

 bedded. I selected several samples which seemed to me to 

 be most characteristic. The first examined was a highly 

 crystalline rock of a dark greyish black colour, and breaking 

 with an irregular fracture. 



In a thin slice I observed it to have a peculiar and 

 beautiful structure, and one which I have also found in other 

 hornfels rocks in parts of the Dargo area. The original 

 structure has been completely obliterated. If it contained 

 any clastic quartz grains in its unaltered state, such are not 



