ABSTRACT
This report contains a detailed micro-minerological analysis of an altered volcanic ash from the upper Niobrara Cretaceous of Logan County, Kansas. Its identity as an ash is positive, but it is not a true bentonite such as is common in rocks of the same age elsewhere in the west.
The ash extends throughout a considerable area, 2,000 square miles, with remarkable uniformity. It extends through 200 feet of columnar section. Deposition was made during a period of very quiet seas, as evidenced by little disturbance of the thin beds.
The several beds are of use in making a structure map of the area, as they are nearly parallel to the Niobrara-Pierre contact.
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