Pleistocene to Pre-pleistocene of Mississippi Basin. 365 



generally, of coarse yellow and orange sands, with everywhere 

 more or less coarse gravel, and has usually a layer of white or 

 variegated clay at the base. The gravel is ... . sometimes 

 cemented by oxyd of iron into great blocks of coarse con- 

 glomerate."* Setting aside the question as to subdivisions of 

 the gravels and sands and clays, which the geologists of Illinois 

 have seen no good reason for separating, f it is true, generally 

 speaking, that the body of the gravel overlies the sand. More 

 accurately, the gravel predominates above and the sand below, 

 though the gravel is to be found more or less generally dis- 

 tributed through a considerable vertical range, in the formation 

 in southern Illinois. 



Within this series, there are often seams and pockets, or 

 even considerable layers of kaolin-like clay, remarkable alike 

 for its irregularity of development and its brightness and va- 

 riety of coloring. Oftener than otherwise, the clay is in the 

 lower part of the sand and gravel stratum, but many localities 

 are known to the writers where heavy beds of gravel and sand 

 occur below a considerable thickness of clay. In such cases 

 at least, the clay would seem to be of necessity correlated with 

 the sands and gravels. 



At several horizons in various localities, the sand becomes 

 more or less coherent from the presence of a line sticky matrix 

 of earthy material. The matrix may even predominate over 

 the sand, giving rise to a gritty clay or loam, rather than to a 

 sticky sand. Such gritty clays are wholly unlike the kaolin- 

 like clays above noted. The latter are gritless and variegated 

 in color. The clays here designated as gritty, have always a 

 goodly proportion of coarse sand, and are either of a deep red 

 or a brownish color. In these gritty clays there are frequent 

 laminae of sand, of variable thickness, and these laminae are 

 often cemented, forming layers of sandstone of considerable 

 hardness. The same is frequently true of laminae of sand, and 

 even of layers of considerable thickness, within the body of 

 the sand itself, where there is little admixture of earthy ma- 

 terial. In all these cases iron oxidp is the chief cement. 



Induration is not confined to the sand. Not rarely portions 

 of the gravel are cemented into a firm conglomerate, the 

 cementing material being the same as in the case of the sand. 

 More commonly than otherwise, it is the uppermost portion 

 of the gravel which has been indurated, but there are so many 

 exceptions that this can hardly be said to be the rule. The 

 thickest beds of conglomerate which have been seen by the 

 writers (near Metropolis, 111.) occur at a low horizon, and 

 (though not shown in vertical section) beneath considerable 

 thicknesses of sand, clay, and loose gravel. 



* Safford, this Journal, vol. xxxvii, page 371, 1864. 

 f Engelman, vol. i, Geol. Surv., lii, page 423. 



