THE PERCENTAGE COMPOSITIONS OF ROCKS 



2bl 



may become necessary to cut off some of the illumination from 

 the paper. This, of course, is done by inserting the blue glass 

 screens provided with the camera lucida. In other cases, when 

 the light from the stage is too bright, the blue glass may be swung 

 to that side, or, more simply, a blue glass may be placed over 

 and parallel to the thin section and below the objective, or some 

 of the light may be cut oft" by holding a finger before the mirror, 

 by tilting it, or by partially closing the lower diaphragm. In 

 some cases one method is best; in others, another. Sometimes 

 the drawing or the mineral is made more distinct by simply holding 

 the eye at a greater distance from the camera lucida. 



THE BUTTE QUARTZ MONZONITE MEASURED BY PLANIMETER 



The writer uses volume percentages for his determinations 

 instead of weight percentages. For example, a rock is half-light 

 and half-dark to the eye when the volumes are 50-50 and not 

 when their weight percentages are 50-50. Thus a rock half 

 plagioclase and half magnetite by volume would have the propor- 

 tions 34.4 to 65.6 by weight. In the following table, therefore, 



*The volume percentages by the planimeter method here given are not to be compared with the weight percentages in 

 the paper by Johannsen and Stephenson in the preceding number of this Journal, since they represent a single and not 

 typical slide; namely, the one shown in the first column of Table III in that article. 



volume percentages are used. The first four columns give the 

 Rosiwal readings made by four different students on the same 

 slide of Butte quartz monzonite; the fifth column is their average. 

 The sixth column is the average of eight planimeter readings on 

 four fields of the same slide. The similarity of each pair of read- 

 ings is to be noted. The differences shown by the four fields are 



