Augusts, 1898.] 



SCIENCE. 



147 



still under investigation, I likewise guarded 

 myself against expressing the opinion that 

 all the so-called Potomac deposits were 

 Jurassic. My words on this point were as 

 follows : 



" It cannot, of course, be positively as- 

 serted at present that the entire series now 

 known as Potomac is all Jurassic, or repre- 

 sents the whole Jurassic. The Lias appears 

 to be wanting, and some of the upper strata 

 may possibly prove to belong to the Da- 

 kota."* 



THE DAKOTA SANDSTONE. 



In regard to the sandstone known as 

 Dakota, and generally considered of Cre- 

 taceous age, I spoke cautiously, as behooves 

 anyone who has seen this formation at 

 many of its outcrops over a wide range of 

 territory in the West, where its phj'sical 

 characters are striking and its fossil re- 

 mains are mainly detached leaves of plants. 



In figure 1 of my paper, showing geo- 

 logical 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 Da- 

 kota, exactly where I had found a sand- 

 stone, 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 Da- 

 kota in my section I consider as more ex- 

 tensive than the single series of sandstones 

 defined as Dakota by Meek and Hayden in 

 1861. The original locality of this sand- 

 stone was the bluffs near the Missouri 

 Eiver in Dakota County, Nebraska, and 

 these authors included with this the sup- 

 posed southern extension of the sandstone 

 in eastern Kansas. This placed the Da- 



* Science, Vol. IV., p. 807, 1896. 



kota on the eastern margin of the great 

 Cretaceous basin which extended westward 

 to the Eocky Mountains. The attempt of 

 Meek and Hayden to identify the Dakota 

 further north, near the mouth of the Judith 

 Eiver, is now known to have failed, but 

 the name transferred to certain sandstones 

 along the flanks of the Eocky Mountains 

 has been accepted, and this term has long 

 been in use for these strata from Canada to 

 Mexico. With this so-called Dakota sand- 

 stone, however, have been included other 

 deposits, the upper part of which may be 

 Cretaceous, while the rest I regard as Ju- 

 rassic, and with good reason. These inter- 

 mediate beds may be seen at various places, 

 especially around the border of the Black 

 Hills and along the eastern flanks of the 

 Eocky Mountains in Colorado. As I shall 

 refer to this point later in the present com- 

 munication, I will not discuss it here. 



OPINIONS OF VAEIOtrS GEOLOGISTS. 



The paper I have now cited I regarded 

 as the preliminary statement of an im- 

 portant case, and not its final demonstra- 

 tion. When presented to the Academy it 

 received the general approval of the mem- 

 bers 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 conclu- 

 sions, and added that he himslf had long 

 suspected that the strata under discussion 

 would prove to be of Jurassic age. 



When an abstract of my communication 

 was published, although without the main 

 illustrations shown to the Academy, I re- 

 ceived further endorsement from geologists 

 familiar with the subject, but from others 

 marks of disapproval predominated. This 

 I had anticipated in a measure, especially 

 from the paleobotanists, whom I believed 

 responsible for much of the confusion that 

 had so long delayed the solution of similar 

 questions. East and West. This point I 



