570 



PRINCIPLES OF STRATIGRAPHY 



tains a higher percentage of ferric oxide than that found in these 

 red kitytes. Thus of seven analyses of North American loess, the 

 lowest gave FeoOs 2.52% and FeO 0.31% and the highest FcoOg 

 5.22% and Feb 0.35%. Another had FcoOg 3.74% and FeO 

 1.02%. The average of the seven is 3.35% Fe.Og and 0.56% FeO. 

 (Clarke-io: ^(5'd.) It is thus seen that so far as the red color of 

 the Vernon shale is concerned it is due only to the anhydrous con- 

 dition of the ferric oxide, and not to the greater abundance of this 

 mineral. 



Fig. 119. Cross-bedding in Sylvania Sandstone (Siluric). (After Sherzer 

 and Grabau). Scale i :6o. 



Older Deposits of Dune Type. 



Consolidated deposits showing the characters and structures of 

 dune sands have been obtained from many of the geological hori- 

 zons of the past, and closer attention to the structural details of 

 rock masses will probably reveal their presence in still other forma- 

 tions. Among Tertiary sandstones which have bee a regarded as 

 of this origin or show evidence pointing that way may be men- 

 tioned the Monument Creek beds of Colorado, while an example of 

 Cretacic sandstones of this type is found in the Nubian sandstone 

 of Egypt, though this is probably not of this character throughout. 

 Certain parts of the Dakota sandstone of the western L'nited States 

 and of the Raritan formation of the east also must be mentioned. 

 Other examples will undoubtedly be found on closer study. In 

 the Jurassic of western North Ainerica occurs a sandstone of great 

 thickness and wide areal extent, which shows in a wonderful man- 

 ner the peculiar type of cross-bedding formed by migrating sand 



