50 GEOLOGICAL SURVEY OF THE TERRITORIES. 



DAKOTA GROUP. 



Although we have hitherto regarded this as a distinct group of Cre- 

 taceous rocks, with a strong physical line of separation from the group 

 above, I now think the evidence is clear that it passes imperce{)til)iy iu 

 its lithological relations up into the Fort Benton group, without any 

 break in time. 



In the hills back of Dakota City there are repeated exposures which 

 show the transition layers between the two sandstones of the one aud 

 the dark plastic clays of the other. The fossils, however, so far as we 

 now know, are distinct, and for the practical purpose of investigatiug 

 this rock they may be regarded as distinct groups. 



We have referred the rocks of the Dakota group to the Cretaceous 

 epoch from the fact that they have yielded numerous species of dicotyled- 

 onous leaves. Among these leaves. Dr. Newberry and Professor Heer 

 have identified those of trees belonging to the genera PopuluSj (poplar,) 

 /Salix, (willow.) Alnus, (alder,) Platanus, (sycamore,) Liriodendron, (tulip,) 

 Ficus, (fig,) and many others. In the history of geology no dicotyledo- 

 nous leaves have been found in fossil condition in rocks older than the 

 Cretaceous era. As they are found here in beds lying underneath rocks 

 containing well-known Cretaceous fossils, their age is beyond a doubt. 



The discovery of these vegetable impressions in sandstones of the Cre- 

 taceous period at this locality has afibrded to geologists an instructive 

 lesson. A geologist of high character, and one of the best botanists in 

 Europe, Professor Heer, declared these plants to be of Tertiary age, and 

 even identified some of them with plants already known in the Old 

 "World in the Tertiary rocks. 



In 1863 Professor Capellini, of the University of Bologna, Italy, and 

 Professor Marcou, of Switzerland, made a journey up the Missouri River 

 to study these rocks, and to settle this vexed question. 



The results of their labors were i^ublished in the French and Swiss 

 geological journals. The article of Professor Capellini was first trans- 

 lated by me in this country for Silliman's Journal. 



Professor Capellini, in a short but very interesting article, confines his 

 observations mostly to the rocks of the^Dakota group, and remarks that 

 he does not hesitate to regard the observations of American geologists 

 as entirely just. The following remarks close the article of Prolessor 

 Capellini: 



" After all we have observed in relation to the environs of Sioux City, 

 it is easily seen that a stratigraphic series so complete, throws a clear 

 light upon the isolated facts first noticed at Tekauiah and Blackbird 

 Hill, and indicates the exact position of the rocks with dicotyledonous 

 leaves, analogous to the Tertiary leaves of Europe, but belonging in re- 

 ality to the Chalk. 



"It may be estimated that the thickness of these Cretaceous strata in 

 the environs of Sioux City is about forty meters. They may be divided 

 into two distinct j^arts, one rich in leaves, a fresh-water format on ; the 

 other truly cbalky, with fishes and Inoceramus of marine origin. 

 Both are probably not older than the chalk of Maestricht. 



"This has been my opinion from the time I admitted that the dicoty- 

 ledonous leaves of the Big Sioux and Tekamah were Cretaceous. 



"Once the age of the Molasse with leaves established by the aid of 

 the stratigraphy and the animal fossils, it would be interesting if it were 

 possible to arrive at the same results by the vegetable remains. On this 

 account Professor Heer came to my aid and investigated the specimens 

 I collected in my explorations. More than a dozen species were recog- 



