value, corresponding to an intermediate gray in the black-to-white neutral scale. 

 Now if white is added, the brightness, also, is heightened, but the intensity is 

 lowered because of the diluting effect of the white. 



The value of a color — and only the value — may be referred directly to the 

 neutral scale, black to white. A medium red corresponds to medium gray; a 

 very light red (pink), to pale gray. The word tone is used when speaking of 

 the brightness or darkness of grays; values refer to colors. 



If the above facts are kept in mind, it is not difficult to duplicate rock colors 

 by the use of very few pencils. The color maroon, which is often used to 

 describe the color of certain shales, is simply a red hue of medium intensity and 

 low value. To duplicate this color, one should apply a strong red color to the 

 log. The value is raised because of the white paper (or the high value of the 

 pencil) ; so gray of lower value than the red is added (pencil of about 2H 

 hardness). The pink-lavender of some clays and shales employs the same red, 

 applied lightly, but followed with a pencil of about 7H hardness, a light tone of 

 gray. 



Browns are composed of a mixture of the three primaries, plus gray. Since 

 blue and yellow make green, it follows that brown is made also by mixing red, 

 green, and gray. Different browns are obtained by emphasizing the intensity 

 of one or more of the constituent colors; thus a red-brown emphasizes the red. 



In pure, intense pigments, the three primaries cancel out when mixed in 

 the right proportions, and the result is black. The colors in pencils are generally 

 of such low intensity that black cannot be attained with them. 



While it is not practicable for the sample examiner to match all rock colors 

 by mixing red, blue, yellow, and gray, the principles discussed should be learned 

 and applied. With a nominal amount of practice, the rock color can be duplicat- 

 ed within small limits with perhaps only eight pencils. It is often faster to mix 

 two colors than to search for the right pencil. In a series of alternating orange-red 

 shales and sandstones, yellow is first applied to the entire succession. The red 

 is then used over the shale intervals to produce the desired color. Tan-gray 

 shales and sandstones are colored in the same manner, except that a hard lead 

 pencil is run over the yellow. Blue-gray shales and carbonates are made by 

 running gray pencil over the blue in shale intervals. Many time-saving com- 

 binations will be discovered during the course of the work when the principles 

 discussed are applied. Obviously, better work and higher production are the 

 reward for acquiring a working knowledge of color, so that principles guide 

 the worker in the use of this important rock character. 



ABBREVIATIONS It is necessary to use many abbreviations in 



FOR LOG STRIPS the lettered descriptions of well logs because 



of the limited space on the strip. Since there 

 is no standard for the abbreviations of many words, a number of forms are in 



57 



