NOVEMBEE 20, 1896,] 



SGIENGE. 



759 



January, 1888. These were collected by Mr. 

 Hatcher in an iron mine near Muirkirk, Md. , 

 associated with Sequoian cones and silieified 

 wood. This horizon is now known to be the 

 equivalent of the Basal Potomac of Virginia, 

 and a rich flora of ferns, cycads and conifers 

 has been discovered in it by Mr. Arthur Bib- 

 bins, which refers it without doubt to my Eap- 

 pahannock series. From the date of this de- 

 posit to that of the Amboy clays, as I have 

 shown, and a fortiori to that of the Island Series, 

 there was an immense interval of time, and 

 during that interval the flora completely 

 changed. Only 15 species of plants out of 329 

 survived this period,* 



The Potomac formation, therefoi'e, while it is 

 a geological unit, represents a great epoch in the 

 history of the coastal plain, perhaps extending 

 from the Jurassic below to the Cenomanian 

 above, and occupying practically the entire 

 Lower Cretaceous. It is thus to be compared 

 with the Comanche series of Texas, and a mere 

 reference to it without specifying which one of 

 its six great subdivisions afibrds no idea of the 

 position of any fossil that may have been found 

 in it. The two lowest subdivisions, the James 

 Eiver and Eappahannock series, I have called 

 the ' Basal Potomac' It was of this portion of 

 the formation that I treated in a paper read be- 

 fore the National Academy of Sciences in April 

 1888, and of which I said : "If the stratigraph- 

 ical relations and the animal remains shall 

 finally require its reference to the Jurassic, the 

 plants do not present any serious obstacle to 

 such reference."! I still stand by that state- 

 ment, but when it is proposed to refer the 'Newer 

 Potomac' also, with its great dicotyledonous 

 flora allied to that of the Upper Cretaceous, to 

 the Jurassic, the evidence for such a reference 

 must be overwhelming. Indeed it may be 

 questioned whether any amount of evidence 

 from vertebrate remains would be sufficient to 

 convince geologists generally. All geologists 

 know that no dicotyledonous plant has thus far 

 ever been reported with certainty from any for- 

 mation lower than the Cretaceous. In my 



* Fifteenth Annual Eeport U. S. Geological Sur- 

 vey, p. 378. 



fAm. Journ. Sci., 3d. ser.. Vol. XX-XVI., August, 

 1888, p, 131, 



' Sketch of Paleobotany '* this fact was clearly 

 brought out, and in the eleven years of great 

 paleobotanical activity that have elapsed since 

 that paper apeared no discoveries have been 

 made to modify it. It is true that I argued in 

 that paper that the dictyledonous floras then 

 known from the Middle Cretaceous, including 

 our Dakota Group, were too highly developed 

 to warrant the assumption that this class of 

 plants had no earlier origin, and in my diagram 

 (pi. Iviii.) of the probable first appearance of the 

 several great types of vegetation I projected the 

 dicotyledonous column downward into the Jura- 

 Trias The Older Potomac flora was then un- 

 known, and its discovery has gone a long way 

 toward justifying this claim. But in this, as I 

 pointed out in the paper already mentioned, f 

 the dicotyledons are rare, of strange aspect, em- 

 bryonic, and prophetic of the great type of plant 

 life that was to dominate the earth. Even in 

 the Middle Potomac (my Aquia Creek series), 

 which overlies the Eappahannock beds with 

 some indication of unconformity, the dicotyle- 

 dons are peculiar in character and are far out- 

 numbered by the lower forms. Not so the great 

 Amboy Clay flora. Here, and still more 

 markedly the flora of Gay Head and Long 

 Island, the dicotyledons are fully developed, 

 many of them probably belonging to genera 

 now found in our forests. They also make up 

 the great bulk of the vegetation, and ferns, 

 cycads and conifers are comparatively rare. 



In a paper just issued and forming part of the 

 Sixteenth Annual Eeport of the United States 

 Geological Survey I have discussed the ' Earli- 

 est Dicotyledons ' J and also certain * Archetypal 

 Angiosperms, ' § and have reproduced figures of 

 forms from the Jurassic that the late Marquis 

 Saporta thought might represent, not dicotyle- 

 dons, but ancestral angiosperms, prophetic of 

 both monocotyledons and dicotyledons, which 

 he called ' Proangiosperms. ' I sympathize 

 more or less with the point of view of the great 

 French paleobotanist, and fully expect that 

 forms will yet be found in the Jurassic which, 



*Fifth Ann. Eep. U. S. Geol. Survey, p. 441, pi. 

 LVI., LVII. 



tAm. Journ. Sci., 3d. Ser., Vol. XXXVI., August, 

 1888, pp. 129-130, 



JPp, 510-515, ?Pp. 535-536. 



