Age of Metamorphic Rocks, Victoria. 127 



These rocks are well exposed within 200 yards of the outcrops of 

 the schists, which there strike N.W. ^ N. Further down the 

 main valley, by a small cataract, there are some well-exposed 

 schists, again striking N.W. ^ N. 



I could find in the field no evidence whatever of a passage 

 from the slates into the gneiss and schist series, and the strike of 

 the two sets of beds was different. I have, therefore, examined 

 a set of sections of the rocks in order to see if they show any 

 indication of the derivation of the schists from the ordovician 

 sediments. 



III. — The Microscopic Structurk of the Rocks. 



Beginning with the metamorphic series, a rock from the 

 summit of the ridge between Commissioner's Creek and Sawpit 

 Gully may be taken as a typical example. The rock [154]^ 

 examined microscopically consists of alternate bands of quartz 

 and felspar, about 2 mm. and 3 mm. thick, respectively. At 

 more distant and irregular intervals are dark tliin lines. When 

 the rock is examined microscopically the quai'tz appears in bands 

 composed of mosaic. The felspathic material has been broken up 

 into a complex mosaic of fine-grained constituents. Most of the 

 felspar consist of allotriomorphic grains of plagioclase, which are 

 crowded with granules and small prismatic crystals ; both 

 granules and prisms are of the same general character, and both 

 are highly refractive. The small prismatic crystals are recognis- 

 able as zoisite, and the granules are probably of the same 

 material. No orthoclase is determinable. With the zoisite are 

 numerous small flakes of white mica. The dark bands lying 

 scattered through the rock are composed of lines of biotite ; at 

 intervals the lines are thickened by small .segregations of biotite, 

 with which occur some flakes of muscovite. Muscovite is 

 scattered abundantly through the felspathic material, and often 

 occurs along the bands of quartz. The rock has the characters of 

 a gneiss, in which tiie felspar has been broken up into a mosaic 

 of plagioclase, zoisite and white mica. 



Near the junction between the normal metamorphic series and 

 the sediments, both are greatly crushed. On the bed of Twist 



1 The numbers are those of the rock collection of the Melbourne University. 



