The sources of the sediments are not far to seek. As the former land was depressed beneath the sea, its surface was doubtless covered with the products of rock decay, consisting of earths, sands, small bits and larger masses of quartzite. These materials, or at least the finer parts, were handled by the waves of the shallow waters, for they were at first shallow, and assorted and re-distributed. Thus the residuary products on the submerged surface, were one source of sediments.

From the shores also, so long as land areas remained, the waves derived sediments. These were composed in part of the weathered products of the rock, and in part of the undecomposed rock against which the waves beat, after the loose materials had been worn away. These sediments derived from the shore were shifted, and finally mingled with those derived from the submerged surface.

So long as any part of the older land remained above the water, its streams brought sediments to the sea. These also were shifted by the waves and shore currents, and finally deposited with the others on the eroded surface of the quartzite. Thus sediments derived in various ways, but inherently essentially similar, entered into the new formation.

Fig. 7. -- Diagram to illustrate the theoretical disposition of sediments about an island.
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Fig. 8. -- Same as Fig. [7], except that the land has been depressed.
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The first material to be deposited on the surface of the quartzite as it was submerged, was the coarsest part of the sediment. Of the sediment derived by the waves from the coasts, and brought down to the sea by rivers, the coarsest would at each stage be left nearest the shore, while the finer was carried progressively farther and farther from it. Thus at each stage the sand was deposited farther from the shore than the gravel, and the mud farther than the sand, where the water was so deep that the bottom was subject to little agitation by waves. The theoretical distribution of sediments about an island as it was depressed, is illustrated by the following diagrams, Figs. [7] and [8]. It will be seen that the surface of the quartzite is immediately overlain by conglomerate, but that the conglomerate near its top is younger than that near its base.

In conformity with this natural distribution of sediments, the basal beds of the Potsdam formation are often conglomeratic (Fig. [9], Plate [III Fig. 2], and Plate [XXV]). This may oftenest be seen near the quartzite ridges, for here only is the base of the formation commonly exposed. The pebbles and larger masses of the conglomerate are quartzite, like that of the subjacent beds, and demonstrate the source of at least some of the material of the younger formation. That the pebbles and bowlders are of quartzite is significant, for it shows that the older formation had been changed from sandstone to quartzite, before the deposition of the Potsdam sediments. The sand associated with the pebbles may well have come from the breaking up of the quartzite, though some of it may have been washed in from other sources by the waters in which the deposition took place.