THE CRETACEOUS RIM OF THE BLACK HILLS. 265 



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, there- 

 fore, 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 remem- 

 bered that the flora of the Dakota Group, one of the richest fos- 

 sil floras of the world, consists, as now published, of 460 species, 

 of which 429 are Dicotyledons. There are only 6 ferns, 12 

 cycads, i 5 conifers and 8 monocotyledons. The cycads are only 

 known by fragments of fronds or pinnae, 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 diffi- 

 cult 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 strati- 

 graphical 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 



