Electric and similar logs are fully discussed in Chapter 14. It is advisable to 

 study this chapter thoroughly before attempting to apply these logs in connection 

 with lithologic determinations. 



MEANING AND Color is generally conceded to be one of the 



SIGNIFICANCE OF COLOR important attributes of a sedimentary rock, 



ranking in importance with composition, 

 texture, structure, and other properties. In the process of teaching (and learn- 

 ing) the distinguishing properties of sedimentary rocks, diverse methods are 

 employed toward attaining higher precision in the determination of these rock 

 properties. Most of the properties have been reduced to specific terms, and 

 standards have been set up to maintain consistent results in determinative 

 work, as, for example, scales of hardness, tables of grain sizes, and percentages 

 of mineral constituents. But the important property of color has received only 

 desultory consideration. A very few workers have done some research on the 

 subject, but the results of this work have failed to reach those who could put them 

 to the most effective use. 



In order to understand the significance of rock color, or even to describe 

 the color, it is first necessary to understand a few of the fundamentals of 

 color itself. 



Colors observed in rocks under the binocular microscope may be consid 

 ered as pigment colors or mixtures of the three pigment primaries — red, yellow, 

 and blue. The rock color, as perceived, may be due to a single mineral whose 

 color is a mixture of the primaries, or it may be caused by a mixture of a 

 number of grains, each having a distinctive color of its own. The over-all effect 

 of such a rock color may be quite different from the color of any one of the 

 constituent grains. A rock whose color is distinctly brown may be seen under 

 the microscope as a mixture of red quartz and feldspar grains and green glau- 

 conite grains. Red and green pencils matching the grain colors will produce 

 the brown rock color when mixed in the same proportions on the log. 



While the following brief discussion does not purport to be more than a 

 mere introduction to one phase of the subject of color, it may serve to arouse 

 sufficient interest to spur further investigation. 



All colors possess three properties : hue, value, and intensity. These qualities 

 are fundamental and must be understood if color perception is to be in any way 

 analytical. The qualities of color mentioned are also known by other names, 

 which are given, together with the definitions. 



Hue 



The hue of a color is its identity with reference to basic colors. Examples are 

 red, red-orange, blue, blue-violet, yellow, yellow-green, etc. Colors such as olive, 



54 



