Reports and Proceedings—Geological Society of London. 135 
shortly on that, the granodiorite (of slightly more acid composition) 
was intruded into the dacite. The microscopic characters of the 
granodiorite, the dacite, the altered dacites, and the gneiss are 
described. In the altered dacites a slight banding or schistosity 
occurs near the contact, ilmenite is changed to secondary biotite by 
reaction with the felspar in the microgranular groundmass, biotite 
is corroded by the attack of the groundmass, and hypersthene is 
altered at its margin to secondary biotite and secondary quartz. 
Finally, minute granules of blue tourmaline occur in the contact 
rocks. All the changes enumerated above are attributed to contact- 
metamorphism 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 groundmass 
similar to, but coarser in grain than, the groundmass 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 thet 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. (Communicated by 
Professor KE. W. Skeats, D.Sc., A.R.C.S., F.G.S.) 
The apparatus described has been designed and constructed by 
the author, for use in the University of Melbourne. It comprises 
a slitting disk of mild steel and two bronze grinding laps, mounted 
on a very substantial wooden table. The disks and laps are each 
10 inches in diameter, and revolve at about 900 revolutions a minute. 
The disks and laps are connected with endless belts, which in turn 
are connected with wheels driven by a 1 horse-power 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 disk. 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. 
