Marsh — Jurassic Formation on the Atlantic Coast. 107 



The Dakota Sandstone. 



In regard to the sandstone known as Dakota, and generally 

 considered of Cretaceous age, I also spoke cautiously, as be- 

 hooves anyone who has seen this formation at many of its 

 outcrops over a wide range of territory in the West, where its 

 physical characters are striking, and its fossil remains are 

 mainly detached leaves of plants. 



In figure 1 of my paper, showing geological horizons and 

 designed especially to represent the succession of vertebrate 

 life in the West during Mesozoic and Cenozoic time, and so 

 defined in the text, I left a blank space above the Jurassic for 

 the Dakota, exactly where I had found a sandstone, regarded 

 as Dakota, in place at many widely separated localities. I said 

 little about the Dakota itself, as I did not wish then to raise 

 questions outside the scope of my paper. 



Had the occasion been appropriate, I might have said that 

 the group termed Dakota in my section, I consider as more 

 extensive than the single series of sandstones defined as Dakota 

 by Meek and Hayden in 1861. The original locality of this 

 sandstone was the bluffs near the Missouri River in Dakota 

 County, Nebraska, and these authors included with this the 

 supposed southern extension of the sandstone in eastern Kansas. 

 This placed the Dakota on the eastern margin of the great Creta- 

 ceous basin which extended westward to the Rocky Mountains. 

 The attempt of Meek and Hayden to identify the Dakota 

 further north, near the mouth of the Judith River, is now 

 known to have failed, but the name transferred to certain sand- 

 stones along the flanks of the Rocky Mountains has been ac- 

 cepted, and this term has long been in use for these strata from 

 Canada to Mexico. With this so-called Dakota sandstone, 

 however, have been included other deposits, the upper part of 

 which may be Cretaceous, while the rest I regard as Jurassic, 

 and with good reason. These intermediate beds may be seen 

 at various places, especially around the border of the Black 

 Hills and along the eastern flanks of the Rocky Mountains in 

 Colorado. As I shall refer to this point later in the present 

 communication, I will not discuss it here. 



Opinions of Various Geologists. 



The paper I have now cited, I regarded as the preliminary 

 statement of an important case, and not its final demonstration. 

 When presented to the Academy, it received the general 

 approval of the members interested in the subject, and one of 

 them, the late Professor Cope, who was best qualified to weigh 

 the evidence of paleontology, fully endorsed my conclusions, 

 and added that he himself had long suspected that the strata 

 under discussion would prove to be of Jurassic age. 



