Marsh — Jurassic Formation on the Atlantic Coast, 115 



interesting from the fact that many of them are found imbed- 

 ded in the original soil in which they grew, thus marking a 

 definite horizon, the age of which has been ascertained by 

 independent testimony. 



On the Atlantic border of this country we have a corre- 

 sponding horizon, determined to be such by its position and by 

 the vertebrate fossils it contains. At various localities in this 

 horizon, especially in Maryland, Cycad trunks have long been 

 known, and within a few years numbers of very perfect speci- 

 mens have been found under circumstances that serve to fix 

 the horizon in which they occur, and confirm the evidence as 

 to its geological age. 



In the Rocky Mountain region, especially around the margin 

 of the Black Hills, a definite horizon likewise exists, in which 

 great numbers of Cycad trunks are found in remarkable pres- 

 ervation. These Cycads resemble most nearly those from 

 Maryland, found in what I term the Pleurocoelus 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 Dakota, but, 

 as I have already shown in the present paper, the beds so 

 termed in the Rocky Mountain region are not the equivalents 

 of the original Dakota, and some of them are evidently 

 Jurassic. Until recently the Cycads of the Black Hills, 

 although of great size and remarkable preservation, have not 

 been found actually in place. In the large collection of Cycads 

 belonging to the Tale 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 sug- 

 gests that it is essentially the same as that of the Cycad beds 

 in Maryland, which I regard as a near equivalent of the well- 

 known Cycad horizon 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 impor- 

 tant evidence against the view I then maintained, but much 

 additional 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 

 further south, and around the gulf border to the southwest, 

 where new evidence is now coining to light. 



* W. H. Reed, formerly my assistant in the West, informs me that he has 

 found Cycads in the Jurassic of Wyoming, both in the Freeze Out Hills, and also 

 near the Wind River Range. In the former region, they occur about forty feet 

 above the Baptanodon beds, so named from a genus discovered by Mr. Reed. 



