248 



O. W. WILLCOX 



positive. Where the concretions are absent the ferruginous coloring- 

 matter is in general more or less evenly distributed through the sand; 

 in the vicinity of the concretions the red color is much less pronounced, 

 and may even disappear altogether, leaving the sand clean and white. 

 This at least goes to show that the supply of cementing material was 

 local. Further, many of the isolated concretions, compound as well 

 as simple. He in sheaths of puie white sand, which they have decolor- 

 ized as decaying roots might have done. Fig. 7 illustrates a 

 phenomenon often observed in the Redbank formation. There is here 



shown a segregation of 



01 1 ^*^^~^) ^^^ '^^^^ ^^^° more or 



^~~f / ^^^ /^ ^^^^ parallel bands, 



T^ />~— -'t^^r^ which locally give the 



^~' ■ ' ^J sand a prominent but 



^^ ^^ ^ , ^ deceptive appearance of 



Q^l ^^S I CI being finely stratified. 



\^^ ^kr ^ /"^^^ ^^"^^ However, the bands an- 



^^ astamose in a manner 



which shows that they 



are not the result of 



ordinary processes of 



bedding. The sand of 



the light bands is 



poorer, that of the dark bands richer, in iron than sand where 



segregation has not occurred. The iron oxide in the dark bands 



is sufficient in amount to occasion differential weathering, but real 



cementation has not yet taken place; the sand is still so loose that 



it may readily be excavated with the bare hand. These segregations 



probably represent a first concentration of the iron by the mutual 



attraction of like particles. In the sand-bank not far from where the 



photograph was taken is a large nest of concretions which have 



absorbed to themselves the iron of all the bands in their immediate 



vicinity. 



It is not maintained that convection currents have never circulated 

 through the sand, but it is clear that they must have been quite sub- 

 ordinate when and where the concretions were actively forming. 

 Finally, a study of the interior structure of numerous compound 



06 



Fig. 5. — Cross-sections of selected concretions. 



