36 Field Columbian Museum — Geology, Vol. III. 



tions originate at any considerable depth below the surface. The 

 concretions are irregular, lumpy forms without approach to any 

 regularity or symmetry beyond the fact that the majority of them 

 are more or less flattened and many have one flat side. They are 

 occasionally penetrated by minute cylindrical holes up to 2 mm. 

 in diameter such as would be the case if they had been penetrated by 

 rootlets. They are of reddish-brown limonite color rarely approaching 

 a hematite-red in places. They are but slightly consolidated and 

 may be readily reduced to their constituent sand grains by pressure of 

 the fingers. They do not commonly exceed 5 centimeters in 

 any dimension. In composition they are dune sand cemented by a 

 small proportion of limonite which does not fill the voids between the 

 grains. The limonite is merely a coating on the sand grains. When- 

 ever the grains touch their coatings' coalesce, thus cementing the 

 sands into a concretion. There is no evidence of any nucleus in any 

 of the specimens examined nor is there any determinable concentric 

 structure. 



There is no mystery about the origin of these forms beyond the 

 determination of which of three or four common agents has been the 

 predominant precipitant of the cement. The sand of the dunes in 

 which they were found is, like nearly all dune and beach sand, of a 

 yellowish -brown color. This color is due to a thin coating of limonite. 

 Where the, dunes have not been fixed by vegetation, this color is not 

 noticeably lighter at the surface than it is in depth. Where a dune is 

 fixed by vegetation a light sod often forms over the surface. Under 

 this sod the sand is much lighter in color for a depth of a few inches 

 than it is at greater depth. Hence it is to be inferred that the 

 organic compounds derived from the vegetation have, as is cus- 

 tomary, dissolved the iron oxides from that sand which lies immediately 

 under the sod. From organic compounds containing iron dissolved in 

 the so-called humus acids, the metal is rapidly precipitated by any 

 one of several agents, the more common of which are spontaneous 

 changes in the organic solvent, bacterial action, oxidation and hydroly- 

 sis. The hydrated ferric oxide precipitated is deposited by preference 

 as a ' film upon the surface of the sand grains and by spontaneous 

 dehydration forms the limonite cement. 



As he precipitation has fol'owed so immediately on solution as 

 to produce concretions within a few inches of the surface it is pro- 

 bable that the precipitating agent is either air in the pores between 

 the sand grains, iron-secreting bacteria, or more probably a hydro- 

 lization of iron compounds of weak organic acids consequent upon 



