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dominant and characteristic ingredient. The second consists, on 

 the other hand, of a dark-blue clay, mingled with more or less 

 siliceous sand. This latter material constitutes the usual floor 

 upon which the true greensand deposit resls ; and it occurs, in 

 like manner, especially in the northern and eastern portions of 

 Monmouth county, both above the uppermost visible greensand, 

 and included between its beds in one or more alternations. 

 Appearances would seem also to indicate that these two deposits 

 replace each other in the same bed, when traced for considerable 

 distances. Indications of this passage from the one material to 

 the other are chiefly discoverable in the lower portions of the 

 formation, or along its northwestern margin. As the same dark 

 clay, associated with the greensand, abounds throughout the 

 upper portion of the next inferior division of the series, or that 

 which we have styled the potters' clay formation, it is impossible 

 distinctly to define the lower limit of the true greensand forma- 

 tion. I shall, for convenience sake, however, group all the strata 

 below the lowest bed of greensand, with the division containing 

 the potters' clay. 



The external characters, and usual chemical composition of 

 these blue astringent clays lying adjacent to the greensand, have 

 been described in the previous section, when treating of the lower 

 part of the series, to which deposits they more strictly belong 

 than to the greensand. I shall confine myself, therefore, in this 

 section to a description of that part of the present formation 

 which mainl}^ characterizes it, namely, the greensand deposit 

 itself. 



Composition of the Greensand. 

 Description. — The predominant and often the sole ingredient 

 in this bed, is a peculiar mineral occurring always in the form of 

 small dark granules, about the size of grains of gunpowder, 

 their form is roundish, and they are often composed of two or 

 three smaller ones united together, a distinctive feature, by which 

 they may at once be recognised from other dark kinds of sand. 

 Though they contain on the average nearly fifty per cent, of silica, 

 they are not gritty, but may be readily bruised between the teeth, 

 or upon the nail, and some varieties, when moistened, admit of 

 being kneaded into a half plastic mass, like impure clay. The 



