In the rapidly shallow- ' ing sea that covered most of Nebraska's central plains, the Niobrara, the Pierre, and other shales were laid. Much of this part of the ocean for long year.-,, probably ranged in depth from one hundred to two hundred

fathoms. There the little grains of glauconite occurred from decomposition of organic matter contained in tiny foraminiferal shells. This hydrous silicate of potassium and iron is seventeen percent potash. The soil of Nebraska is fertile as a result.

There came a time when the ocean floor was bared, except for pools, lagoons and marshes, and long lakes of slowly moving, brakish water ; and the antecedents of the Niobrara, White river and the Platte ran westward from the mountains to an inland sea. It was at this time, after the Pierre shales were laid, that Hartville island sank, and Nebraska's sea was shallowed. Islands and banks of mud, sand and rock arose dripping from a dismal swamp, and miles and miles of marsh appeared. The Laramie, or Fox Hills, massive sands and varigated shales, and thin silicious lime rocks were laid about the base of the sinking Hartville island. Cross currents made mixed bedding, and slightly moving water left sandstones marked with ripples. Paleo-zoologists say the Laramie period was the last of the Cretaceous, and paleo-botanists say that it was the first of the Tertiary. Marine animal life lingered over into the new and marshy conditions, while plants changed quickly, and the old varieties passed away.

Quite likely, the Cretaceous was before and the Tertiary after, and the Laramie during the Rocky Mountain revolution. It was the period of transition. Benton oysters found new expansion, then changed into large fresh water clams, ten inches long. Soft woods of prodigious growth, that made ligniteous coal, passed away, and hard woods took possession of the plains.