August 5, 1898.] 



SCIENCE. 



153 



As bearing directly on the question here 

 discussecl, the Cycads of the Jurassic period 

 afford instructive examples of the evidence 

 that may be derived from fossil plants 

 under favorable circumstances. The Cycad 

 trunks of the upper Jurassic of England 

 have long been known, and are especially 

 interesting from the fact that many of them 

 are found imbedded in the original soil in 

 which thej^ grew, thus marking a definite 

 horizon, the age of which has been ascer- 

 tained by independent testimony. 



On the Atlantic border of this country 

 we have a corresponding horizon, deter- 

 mined to be such by its position and by the 

 vertebrate fossils it contains. At various 

 localities in this horizon, especially in Mary- 

 land, Cycad trunks have long been known, 

 and within a few j'sars numbers of very 

 perfect specimens have been found under 

 circumstances that serve to fix the horizon 

 in which they occur, and confirm the evi- 

 dence as to its geological age. 



In the Rocky Mountain region, especi- 

 ally around the margin of the Black Hills, 

 a definite horizon likewise exists, in which 

 great numbers of Cycad trunks are found in 

 remarkable preservation. These Cycads 

 resemble most nearly those from Mary- 

 land, found in what I term the Pleuro- 

 cceIus beds of the Potomac formation. In 

 the Black Hills the age of the horizon has 

 not been accurately determined, but present 

 evidence points to its Jurassic age. The 

 strata here containing these characteristic 

 fossils has long been referred to the Da- 

 kota, but, as I have already shown in the 

 present paper, the beds so termed in the 

 Eocky Mountain region are not the equiva- 

 lents of the original Dakota, and some of 

 them are evidently Jurassic. Until re- 

 cently the Cycads of the Black Hills, al- 

 though of great size and remarkable preser- 

 vation, have not been found actually in 

 place. In the large collection of Cycads 

 belonging to the Yale Museum a few have 



been discovered apparently where they 

 grew, and systematic investigation will 

 doubtless show that the various localities 

 where these fossils have been found around 

 the Black Hills are all in one horizon. 

 The evidence now available indicates its 

 Jurassic age, and suggests that it is essen- 

 tially the same as that of the Cycad beds 

 in Maryland, which I regard as a near 

 equivalent of the well-known Cycad hori- 

 zon in the Purbeck of England. 



In conclusion, I have only to say that 

 the year which has passed since my first 

 communication to the National Academy 

 on the Jurassic of the Atlantic border has 

 brought no important evidence against the 

 view I then maintained, but much addi- 

 tional testimony in its favor, especially in 

 the region north of the Potomac River that 

 I then discussed. I still hope to return to 

 the subject later and take up the question 

 of the extension, of the same formation 

 along the Atlantic coast farther south, and 

 around the Gulf border to the southwest, 

 where new evidence is now coming to light. 



Postscript. 



After the preceding article was in print I 

 received some information about Cycad ho- 

 rizons in Wyoming that bore directly on the 

 question I discussed near the end of my 

 paper. This information is of so much in- 

 terest that I add a postscript to place on 

 record the important discovery by "W. H. 

 Reed of two new Cycad localities in the Ju- 

 rassic of Wyoming, both much farther 

 west and quite distinct from those already 

 known around the Black Hills. One is in 

 the Freeze Out Hills of Carbon county, and 

 the other near the Wind River range. 



Mr. Reed has since sent me a more com- 

 plete account of the first of these lo- 

 calities, with a sketch showing the sec- 

 tion of the strata where the Cj'cads were 

 found, and also measurements of the suc- 

 cessive strata exposed, from the Trias 



