THE PERCENTAGE COMPOSITIONS OF ROCKS 



277 



The method described below is based on surface measurements. 

 As will be shown, these are proportional to volumes in any uni- 

 form, non-banded rock, irrespective of the shape of the individual 

 components. 



Place a camera lucida over the eyepiece of the microscope and 

 block up a drawing-board at the side so that the field of view is 

 not too large, say four or five inches in diameter. Tilt the micro- 

 scope or incline the plane of the paper until the drawing is not 

 distorted. This may be determined by tracing the projection of 

 a circle or square upon the paper and measuring it in several 

 directions. If no object-glass with circle or square is at hand, 

 place an ordinary object-scale successively vertically at the right 

 and left of the field of view, and hori- 

 zontally at the top and bottom, and lay 

 off equal distances in the projection. If 

 equal divisions on the scale are equal 

 everywhere in the drawing, the plane of 

 projection is at the proper angle or the 

 microscope is properly tilted. 



Place one edge of a thin straight-edge 

 of wood, celluloid, or cardboard on the 

 drawing-board along the projection of 

 the vertical cross-hair as seen through 

 the camera lucida, and fasten it with 

 thumb tacks. This is to serve as a guide 

 for the stylus of a planimeter, as described Fig. i 



below. Instead of a straight-edge, the 



writer uses a piece of celluloid about i mm. thick, in which is cut a 

 semicircle (Fig. i) exactly the size of the field of view (4^ inches 

 in his microscope with a 31-mm. objective). 



Select the objective most suitable for the particular rock sec- 

 tion in hand; the lower the power, the better. Place a typical 

 area in the field of view, and make on the cover-glass, on the line 

 of the vertical cross-hair, a mark in red ink to indicate the 

 starting-point. Place a planimeter on the drawing-board in such 

 a position that the base will not interfere with the projection of 

 the section as seen through the camera lucida. 



