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From these several facts, viz.: the horizontal strata, the ripple-marks on the surfaces of the layers, the fossils, the character of the sand, and the water-worn pebbles and bowlders of the basal conglomerate, positive conclusions concerning the origin of the formation may be drawn.

The basal conglomerate of the Potsdam formation is shown at the lower right-hand corner, and is overlain by sandstone.

The basal conglomerate formed by bowlder and pebble beaches comes to be covered with sheets of sand, and these with layers of mud as the sea becomes deeper and the shore more remote; while deposits of limestone are made when at last no waste is brought to the place from the now distant land, and the water is left clear for the growth of mollusks and other lime-secreting organisms.

The lowest member is a basal conglomerate composed of pebbles derived from the erosion of the dark crumpled schists beneath,—schists which are supposed to be Archean.

The sea encroached on the new land, and spread far and wide a great basal conglomerate and coarse sandstones.