Samples of red beds of the Permo-Pennsylvanian Maroon formation were studied to determine the constituents contributing to the coloration and the relationship of those coloring constituents to the associated mineral grains in the rocks. These samples were of two main types: fine- to coarse-grained, red-colored, micaceous, arkosic sandstones; and red-colored, micaceous shales. Microscopic study disclosed a close association between a red material, which was identified as hematite by x-ray analysis, and a micaceous mineral which by x-ray analysis was found to be either muscovite, illite, or a transitional mineral whose composition ranged between the compositions of muscovite and illite. The composition of this transitional mineral depends upon the degree of alteration of the biotite from which it originated. The hematite occurs mainly between the lamellae of the micaceous material. This hematitic material fills the interstices of the arkoses and makes up the bulk of the red shale. The alteration of the biotite took place after deposition.