In Figure 24-37 the clastic percentages are shown by solid contours. The 

 clastic ratios are shown by dashed lines, and ratio intervals are indicated by 

 shading. The 50-percent contour is also the ratio 1.0 contour. 



In general the two methods are very similar, the principal difference being 

 that the percentage map is contoured on a regular contour interval of 10 percent, 



TABLE 24-1 



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 contour spacing would de- 

 crease 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 percentage method of contouring the 

 relationships between clastic and non-clastic constituents is simpler, more direct, 

 and generally more practicable. 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 generalized, broad regional work. Conversely, where contour points 

 are numerous, and the data reliable, the percentage maps are better. 



Alternation Frequency (Rate) 



Lithofacies maps generally indicate the relative or absolute quantities of 

 the principal lithologies in a section, but they do not reveal the manner in which 

 these lithologies occur; therefore, a section consisting of alternating thin litho- 

 logic units is not distinguished from one composed of a few thick units. This 

 situation is at least partially rectified by employing an alternation frequency or 

 alternation rale map, which may be constructed according to the following pro- 

 cedure. 



The stratigraphic interval to be studied is determined by correlating logs 

 or surface sections over the map area. Next, the number of alternating litho- 



494 



