20 Diorites and Granites of Swift's Creek, 



(2) Mineral Composition and Classification of the 



Kocks. 



In attempting a description of the mineral composition 

 of the various rock masses met with in the district I am 

 describing, it becomes necessary to begin by a division of the 

 rocks into sedimentary, regional metamorphic, igneous 

 (intrusive) and contact metamorphic, and it is in this 

 sequence that I shall consider them. 



(a.) The Sedimentary Rocks. 



This class is represented in the area described only by a 

 small tract of yellow and green slates and quartzose 

 sandstones, in the south of Riley's Creek, and by a narrow 

 zone of indurated slates and sandstones lying outside the 

 rocks in immediate contact with the intrusive diorites 

 and granites. These indurated sediments approach 

 the character of the normal Silurian rocks, of which 

 they are, I doubt not, part. In them the effects 

 of alteration are not so far advanced as to remove 

 them from the category of the sedimentary into that of the 

 metamorphic rocks. The examples examined were collected 

 at Tongeo West, Swift's Creek, where they pass on the one 

 side into the hornfels group, and on the other into nodular 

 argillaceous schists, which are there the least metamorphosed 

 of the Omeo series. The changes which are observable in 

 these indurated rocks are to a great extent parallel with 

 those which I have noted in a series of slices prepared from 

 the passage rocks, which connect the normal Silurian slates 

 and sandstones with the regional crystalline schists at the 

 Upper Dargo River. The normal argillaceous slaty members 

 of the series, for example, are seen to be more or less 

 permeated by a pale green to almost colourless silicate, 

 which in places forms distinct flakes or small patches. In 

 more altered examples there are more numerous flakes, and 

 also somewhat short and stout but often ragged fibres of 

 similar character. The most complete change is found in 

 those rocks which precede the distinctly micaceous beds, in 

 which the ground mass of the rock appears to be a colourless 

 transparent mineral in scales or flakes, which are doubly 

 refracting and suggestive of kaolin ; and throughout this 

 groundmass there are many — often extremely numerous — 

 pale green interlacing fibres, apparently the representatives 



