626 PRINCIPLES OF STRATIGRAPHY 



northern Arabia. The source (57:^5) of the iron is believed to be 

 in the sand itself, as shown by analysis, the coating having formed 

 under the influence of the sun's heat, as the desert varnish forms 

 on the larger pebbles and boulders. This latter, however, is 

 subject to destruction owing to the size of the fragments, for 

 Walther has observed that after a heavy rain this brown coating 

 is quickly removed by the impact of the rock masses. In like man- 

 ner a coating of iron oxide on sand grains subject to wind trans- 

 port must be destroyed, and this probably accounts for the almost 

 uniform white or golden color of desert sands. The absence of 

 such a coating, then, on the pebbles of ancient desert gravels 

 need not be surprising, and the yellow or white color of gravel 

 and coarse sand beds intercalated between red deposits may not 

 necessarily indicate great climatic differences, but may result rather 

 from the destruction of the color coat in the coarser material. 



EXAMPLES OF OLDER CONTINENTAL HYDRO- 

 CLASTICS. 



Examples of fluviatile and lacustrine deposits have been rec- 

 ognized in nearly all geological horizons, from the pre-Cambric to 

 the present. Not all stratigraphers agree in regarding the forma- 

 tions enumerated below as of unequivocally non-marine origin, but 

 the more obviously fluviatile and glacial formations are recognized 

 as such by most recent students of the subject. 



Cenozoic or Tertiary Examples. 



Among the Tertiary deposits of the Great Plains regions of the 

 western United States are many beds showing stratification, but 

 composed in large part of alternate pebble and sand beds, with 

 cross-bedding structure well marked. These have commonly been 

 classed as "lake deposits," but, as Davis (19:5/5) has shown, these 

 are more likely deposits made by running water, and represent 

 outwash plains or alluvial fans, formed by the streams from the 

 mountains. Some of these deposits, as in the case of the Vermillion 

 Creek beds in Wyoming, consist near the mountains from which 

 they have been derived of excessively coarse conglomerates be- 

 tween 3,000 and 4,000 feet thick, nearly structureless, lines of 

 stratification being rarely perceived. "The blocks of which the con- 

 glomerate is chiefly formed range from the size of a pea to masses 

 with a weight of several tons . . ." (King-35 :j(5p.) At some 



