Geology 



their denudation by the East River and its tributaries is 

 now in progress. 



After collecting specimens of basalt from the dyke, 

 and as many of the fossils as we could find, we returned 

 to our camp by the river and dropped down the stream 

 to Smithford, where we rested for a few days before 

 resuming our journey up the Hutton River. The time 

 was profitably spent in getting our floating laboratory 

 into working order and examining and numbering the 

 specimens which we had collected. 



The basalt is a very fine-grained rock, and it is 

 impossible to determine its constituent minerals even 

 with the aid of a pocket lens. This being the case we 

 may proceed to prepare a thin section for examination 

 under the microscope. This is done as follows. First 

 strike off with a hammer as thin a flake as possible, and 

 then rub one side of this on a zinc plate with coarse 

 emery powder until it is quite flat. Now polish this 

 flattened surface on another zinc plate charged with fine 

 emery, and finally on a piece of wash-leather stretched on 

 a board and covered with putty 'powder or jeweller's rouge. 



The specimen must now be cemented by its polished 

 surface to a glass microscope slide (usually 3 x i inches) 

 by means of Canada balsam, which is a transparent 

 cement. When this has become hard, the other surface 

 is first flattened, then ground so thin that light begins to 

 show through it. 



Great care must now be taken, as the thin slice is 

 very fragile, and it must be slowly rubbed down on the 

 plate with fine emery and finally polished on the wash- 

 leather. It should now allow light to pass freely, and 

 if it does so is ready for mounting. 



106 



