The conglomerates of the Galena-Trenton series of the Ordovician have been once before described2 in an article in the American Geologist. At that time those conglomerates were interpreted as intraformational in contradistinction to basal or interformational conglomerate.3 They were interpreted as corrosional in contrast with such as are commonly formed by erosion. It is my purpose now to describe one of those conglomerates in particular and to discuss its significance in relation to other conglomerates. The conglomerate to be described here is a thin or scattering deposit in the form of black-surfaced pebbles of limestone (see plate 12) in a greenish shaly limestone matrix. It occurs above the base of the Galena limestone formation proper. It is best known at Saint Paul, Minnesota, but is seen also at Kenyon, near Mantorville, and in other places where the base of the Galena is exposed.

The mere occurrence of . . .

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