950 



Subsurface Geologic Methods 



of 10 per cent, whereas the intervals of clastic ratios are 0.5, 1.0, 5.0, and 

 10.0. If the ratio-contour interval were constant arithmetically, the con- 

 tour spacing would decrease at an exceedingly high rate toward the higher 

 ratios. Therefore, as Krumbein points out, the contour interval should be 

 determined by a logarithmic or geometric means. For this reason the 



Figure 512. Map showing clastic percentages in solid contours and clastic ratios in 

 dashed contours. Clastic ratio of 1.0 — 50 percent. 



percentage method of contouring the relationships between clastic and 

 non-clastic constituents is simpler, more direct, and generally more practic- 

 able. It should be stated in fairness to the ratio method, however, that it 

 tends to emphasize areas where the clastic constituents form a substantial 

 part of the section; hence it is a good method for some types of very 



