918 Geological Society. 



Finally, minute granules of blue tourmaline occur in the contact- 

 rocks. All the changes enumerated above are attributed to contact- 

 uietamorphism caused by the intrusion of the granodiorite. 



In the gneiss, hypersthene is not found, ilmenite is rare, and 

 the rock is completely foliated. It shows a granular ground -mass 

 similar to, but coarser in grain than, the ground-mass of the dacite. 

 Besides occurring at the contact, it has also been found in parallel 

 zones intercalated in dacite near the contact. 



The author believes that the gneiss is a peculiar modification of 

 the dacite, but direct evidence as to its mode of origin is as yet 

 incomplete. It may possibly be the result of extreme contact- 

 metamorphism of a dacite of peculiar character, such as a tuff. It 

 is possible that it was produced by differential movement in the 

 dacite before complete consolidation, and certainly before the 

 intrusion of the granodiorite. Since, however, dynamic effects are 

 present in some sections, and are accompanied by changes found in 

 the dacites altered by contact-metamorphism, the author is rather 

 inclined to support the view that primarily the gneiss is due to 

 differential movements in part of the dacite series, complicated by 

 effects due to contact-metamorphism by the later intrusion of the 

 granodiorite. 



3. ' Recent Improvements in Rock-Section Cutting Apparatus.' 

 By H. J. Grayson, Demonstrator of Petrology and Assistant in the 

 Geological Department, University of Melbourne. 



The apparatus described has been designed and constructed by 

 the author, for use in the University of Melbourne. It comprises 

 a slitting disc of mild steel and two bronze grinding laps, mounted 

 on a very substantial wooden table. The discs and laps are each 

 10 inches in diameter, and revolve at about 900 revolutions a 

 minute. The discs and laps are connected with endless belts, which 

 in turn are connected with wheels driven by a 1-horsepower electric 

 motor. Special clamps are used to attach the rock-specimen and to 

 cut the slice. A goniometric crystal-holder, permitting of slicing in 

 any desired direction, is described, and can be fitted to one of the 

 clamps. Clamps swinging radially across the grinding laps permit 

 the parallel grinding of the slice to any required thinness. A 

 polishing lap can be placed in the position of one of the grinding 

 laps. The finishing of the slice is done by hand on a slate disc. 

 In the second part of the paper, the author describes in detail the 

 method which he employs in making a rock-section, and refers to a 

 number of improved methods or variations of the usual processes 

 which he has in practice found advantageous. 



