INTRODUCTION. 6 



Colorado. The plants, by the identity of a number of them and the close 

 affinity of character of some others with species of the Dakota Group, 

 have positively confirmed the supposition that this formation, passing 

 westward in Kansas under the Tertiary measures, is prolonged under them 

 and continues to the Rocky Mountains. 



Already, in 1873, Dr. A. C. Peale had procured from Colorado fragments 

 of poorly preserved leaves which had been recognized as identical with 

 Proteoides acuta, Heer, a species commonly found in the Dakota Group of 

 Kansas and Nebraska. From this, Nos. 14^16 of the section of South 

 Platte River^ had been then considered by Dr. Hayden as referable to a 

 Cretaceous formation. More recently, Passed Assistant Engineer H. C. 

 Beckwith, United States Navy, and Rev. Arthur Lakes, have got, near 

 Morrison, a few miles west of Denver, numerous specimens of some of the 

 more predominant species of the Dakota. Group — Sassafras {Aralioj)sis) 

 cretaceum, Magnolia CapelUni, Aralia, Salix proteoefolia, etc., with some 

 others, which though new are related species which tend to identify the 

 Cretaceous formation at the base of the Rocky Mountains with that of 

 Kansas. Admitting, therefore, the prolongation of the Dakota Group 

 under the Tertiary measures to the base of the mountains, the width of 

 the area covered by this formation should be estimated from east to west 

 at 450 to 500 miles. 



Perhaps, also, I should omit here any remarks on the flora of the 

 North American Cretaceous as represented by the plants of the Dakota 

 Group, having already, in Volume VI of the United States Geological 

 Survey of the Territories, by Dr. F. V. Hayden, considered the general 

 character of this flora and its relation to plants living at our time, or to 

 analogous or identical species observed in the formations succeeding that 

 of the Cretaceous. But the materials which I had then for consideration 

 were few and local; they have since been greatly increased, and also new 

 points for comparison have been furnished to phytopaleontologists by the 

 works of Heer on the recently discovered Cretaceous plants of Greenland. 

 From this, some of the conclusions formerly admitted have been more 

 or less modified, while others have received a higher degree of precision 



■ Dr. F. V. Hatdkn, Annual Report, 1873, pp. 195, 196. 



