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SUMMARY OF CURRENT RESEARCHES RELATING TO 



desired. The drive for the lap is taken over jockey pulleys in order to 

 get it horizontal. Mud-guards are fitted to both the lap and the disk, 

 but these are not shown in the illustration. In the actual preparation 

 of the rock a slice is cut from the specimen with the disk, generally 

 about ijV in- thick. This is then smoothed down on the lap to remove 

 the rough saw-cut marks, and is then ground down on a plate of glass 

 with suitable abrasive, I use carborundum No. 150 grit, which I find 

 quite suitable. The rock is rubbed with a circular motion with short 

 sweeps until a perfect plane surface is obtained. It is then rubbed on 



Fig. 2. 



another piece of glass with FF carborundum till the scratches are re- 

 moved, and is then polished off on a piece of Xo. emery cloth. A 

 high polish is not necessary, as long as there are no scratches. The 

 specimen is then mounted on a piece of plate-glass about 1^ in. square 

 to act as a handle for the final grinding. Hard Canada balsam is gene- 

 rally used for this, but I use ordinary gum arabic in a rather thick 

 solution, which I find answers the purpose for all except very friable 

 rocks. When the rock is firmly affixed to the holder, it is ground down 

 on the lap. It is difficult to give any particulars as to the time to be 

 spent on this rough grinding, as it depends on the rock. I usually stop, 

 however, as soon as the edges begin to crumble away, as a few extra 

 turns will take the specimen entirely off the glass. When it is judged 



