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ANNALS NEW YORK ACADEMY OF SCIENCES 



section (Fig. 85). The iron in this case is sometimes, at least, in the 

 form of limonite, rather than hematite. Magnetite is present in small 

 amounts, and dense patches of limonite represent oxidized pyrite. The 

 kaolin is more or less abundant and is mixed with fine-grained carhonitic 

 material. Hematite is also present in small amount. 



The hematite in the red or brown grits has probably originated through 

 oxidization of the siderite present in the light colored calcareous sand- 



m * 



Fig. 87. — Calcareous sandstone, from the lower teds of the Morrison formation at 

 Garden Park, Colorado. 



The clear grains are quartz, and the mottled grains are calcite. About 28 X. 



stones. The iron carbonate was probably present in the original deposits. 

 The alteration may have taken place to some extent before burial, but it 

 is more likely that it is the result of a long-continued process in the 

 buried rock." 



The clays are nearly always impure. They usually contain, along 

 with the kaolin, a considerable amount of fine angular quartz and very 



' See the discussion of the origin of the formation, p. 168. 



