378 



GEORGE B. RICHARDSON 



which the white minerals are muscovite, calcite, magnesite, 

 kaolin, gypsum, and feldspar. Some magnetite and ilmenite and 

 occasional fragments of chlorite, are also present. 



No systematic arrangement of the minerals occurs, the texture 

 being characteristically sedimentary. The individuals are all 

 minute; few are over o. i""° in cross-section, and the average 

 is about 0.04""". 



Magnified 50 diameters. X56 Magnified 136 diameters. 



Micropiiotographs of thin sections of the spearfish red beds. 



X136 



The quartz grains vary in size from O.03 to 0.06™"^ in cross- 

 section. The particles are angular to subangular, seldom well- 

 rounded. Some of the contours are so irregular, though smooth, 

 as to suggest corrosion. Many of the quartz grains are perfectly 

 clear and transparent, while others contain inclusions. Slender 

 prisms of rutile are included in a few of the quartz grains, while 

 others contain fluid inclusions. These do not contain red pig- 

 ment and probably are original minerals derived from the dis- 

 integration of the parent rocks. Other quartz grains contain 

 numerous inclusions of the red pigment and doubtless this quartz 

 is secondary, being formed in the presence of iron oxide. 



Muscovite is present in small clear rods, averaging o.i ^^ long 



