and their Contact Zones. 21 



of the previously remarked diffused silicate. These suggest 

 chlorite, and so far as can be observed of such microliths, 

 become dark when their longer diameter accords with the 

 polarising plane of either of the crossed nicols. The change 

 in the Swift's Creek passage rocks, or least altered of the 

 contact schists, very strongly resembles that seen in the 

 Upper Dargo passage rocks. The ground mass consists of 

 minute, colourless, transparent, overlapping scales or flakes, 

 probably orthorhombic, and in this, either irregularly dis- 

 seminated or lying parallel to the bedding, are great numbers 

 of pale green to almost colourless, rather stout, and usually 

 bent and twisted fibres. So far as it is possible to speak 

 with any certainty of such microliths, it seems to me that they 

 become obscured when their longer diameters are parallel 

 with the plane of polarisation of one of the crossed nicols. 

 It seems to me, therefore, probable that the rock -forming 

 materials of these argillaceous strata have become rearranged 

 as two distinct silicates. In the absence of a quantitative 

 analysis of the rock, it is not possible to speak with any 

 certainty ; but assuming that these Silurian slaty rocks have 

 a composition similar to that of such rocks elsewhere, and 

 which have been analysed, the idea is not unreasonable that 

 in the colourless, probably orthorhombic, scaly silicate we may 

 recognise a mineral allied to kaolin, and in the pale-green, 

 stout, twisted fibres or plates, a magnesian silicate allied to 

 chlorite. Besides these component silicates there is some 

 amount of amorphous, black, opaque material, unaffected by 

 hydrochloric acid, which may probably be carbonaceous. 



Throughout these rocks strings and lenticular groups of 

 quartz grains, as also contorted veins of quartz containing 

 the chlorite fibres (?), indicate metamorphic action. 



Classification of the Sedimentary Rocks. — It is not neces- 

 sary to say much as to the classification of these rocks. 

 That they are sedimentary in origin is shown by their posi- 

 tion and structure in the mass. Their probable geological 

 age is indicated by the general considerations to which I 

 have already referred; and the microscopic examination 

 shows that they are the indurated representatives of rocks, 

 such as the fine-grained aluminous slates and the quartzose 

 sandstones of the Silurian formation of the district. 



(b.) The Regional Metamorphic Rocks. 

 The regional metamorphic schists which are found on the 

 north and west side of Swift's Creek include, I believe, 



