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THE CRETACEOUS RIM OF THE BLACK HILLS.
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Leaves of Pinus and Leptostrobus occur quite frequently in the Potomac formation in Virginia, Alabama and New Jersey, but have never been found in the Dakota Group. So far, therefore, as these forms from the Black Hills go they favor the view that the bed in which they occur is Lower Cretaceous.

The chief argument from the plants is that they were all of humble types, no dicotyledonous leaves occurring among them. The force of this argument may be appreciated when it is remembered that the flora of the Dakota Group, one of the richest fossil floras of the world, consists, as now published, of 460 species, of which 429 are Dicotyledons. There are only 6 ferns, 12 cycads, 15 conifers and 8 monocotyledons. The cycads are only known by fragments of fronds or pinnæ, and a few doubtful fruits. The chances are hundreds to one that any plant bed of that age will contain dicotyledonous leaves in profusion, and the lower forms very sparingly, if at all. This was found to be the case at the real Dakota plant bed above Evans Quarry. Only one fern was obtained, while leaves were abundant though difficult to secure entire with the insufficient appliances with which we were provided.

A closing word on the bearing of these facts upon the Lower Cretaceous of North America may be permitted. It would seem probable that a considerable portion of the deposits underlying the marine Cretaceous of the Rocky Mountain region which have heretofore been referred to the Dakota Group on purely stratigraphical evidence may really be much older. When in 1883 I descended the Missouri River from the mouth of Sun River to Bismarck, most of the way in a "mackinaw," and in company with Dr. C. A. White, that able geologist was of the opinion that the rocks at the Great Falls of the Missouri belonged to the Dakota Group. They were seen distinctly passing under the Fort Benton shales below, and there were no more indications of a division line at any point in the series than Professor Newton found in the same section of the Black Hills. As no Cretaceous older than the Dakota Group was at that time supposed to exist in that region, it was natural to refer all below the Fort Benton to

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