[Illustration: Fig. 7.--Diagram to illustrate the theoretical disposition of sediments about an island.] [Illustration: Fig. 8.--Same as Fig. 7, except that the land has been depressed.] 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. [Illustration: Fig. 9.--Sketch showing relation of basal Potsdam conglomerate and sandstone to the quartzite, on the East bluff at Devil's lake, behind the Cliff house.] The basal conglomerate may be seen at many places, but nowhere about Devil's lake is it so well exposed as at Parfrey's glen (a, Plate XXXVII), where the rounded stones of which it is composed vary from pebbles, the size of a pea, to bowlders more than three feet in diameter. Other localities where the conglomerates may be seen to advantage are Dorward's glen (b, Plate XXXVII), the East bluff at Devil's lake just above the Cliff house, and at the Upper narrows of the Baraboo, above Ablemans. While the base of the Potsdam is conglomeratic in many places, the main body of it is so generally sandstone that the formation as a whole is commonly known as the Potsdam sandstone.